Car AC Not Working: Why a Recharge Is Rarely the Answer

Air conditioning is the one system on a car that almost everybody is content to treat as a consumable. It goes weak across a couple of summers, somebody says it wants gassing up, a garage visit or a can from a shelf puts it right for a while, and the cycle repeats. That model of how the thing works is comfortable, widely repeated and wrong — wrong in a specific way that quietly converts a cheap repair into an expensive one.

Here is the correction, and everything else on this page follows from it. An air conditioning system is a closed circuit. Nothing in normal operation removes refrigerant from it. The charge that was put in at the factory is meant to still be in the car many years later, and on a healthy system it very largely is. So a system that is low has not used something up. It has a hole in it. The recharge that everyone reaches for first is not maintenance; it is a way of putting the hole out of sight for a season.

The other useful thing about this subject is that the complaint itself carries a great deal of information before anybody attaches a gauge to the car. Air that is warm all the time, air that starts cold and gives up after a few miles, no air movement whatsoever, and cold air that arrives with a smell are four different reports pointing at four different parts of the vehicle — and one of them is very often not an air conditioning fault at all. The figure above maps each observable symptom onto what it points at. The sections that follow explain why each one lands where it does, so you can recognise the reasoning rather than memorise the table.

Later sections change reader. They are addressed to whoever is inspecting a car with a view to buying it, rather than to an owner repairing one they already have. Air conditioning holds an odd position in a used-car purchase: among the simplest systems to assess, and among the most reliably skipped, because for half the year the weather hands a seller an entirely innocent reason to leave it undemonstrated.

A sealed loop does not use refrigerant

Start with what the system is actually doing, because the argument depends on it.

Refrigerant is not a coolant in the sense that the stuff in the radiator is a coolant. It is a working fluid that is deliberately pushed back and forth across its own boiling point. The compressor squeezes it into a hot, high-pressure gas. The condenser, out at the nose of the car in the moving air, takes heat out of it until it turns to liquid. That liquid is metered through a restriction — an expansion valve or a fixed orifice tube — into a low-pressure region, where it boils. Boiling absorbs heat, and the place it is made to boil is the evaporator, a small heat exchanger buried in the dashboard with the cabin air blowing across it. The heat in your cabin is picked up there, carried around the loop as latent heat, and thrown out of the front of the car. Then the cycle repeats, with the same refrigerant, indefinitely.

Nothing in that description consumes anything. The refrigerant changes state constantly and is destroyed by none of it. Neither is it burnt, filtered out, or worn away. It is the same working fluid going round and round, and the only mechanism by which a car can end up with less of it than it started with is escape.

So when a system is found to be low, exactly one thing has been established: refrigerant that used to be inside the car is now outside it. Everything else — how fast it went, from where, whether it went through a seal that was always marginal or a hole that has just appeared — is still unknown, and the recharge does not address any of it.

There is a second passenger in that loop, and it is the reason this matters financially rather than merely philosophically. The compressor’s lubricating oil is carried around the system by the refrigerant itself. There is no separate oil pump, no sump, no dipstick. Oil goes where the refrigerant goes. A system that has lost a significant part of its charge has usually lost oil along with it, and a compressor being asked to run on a low charge is a compressor being asked to run on reduced lubrication and elevated temperature. That is the mechanism by which a cheap fault becomes an expensive one: not the leak itself, but the months of driving with the button pressed while the system is starved.

Which produces the single most useful piece of advice on this page, and it costs nothing. If your air conditioning has stopped being cold, switch it off and leave it off until you know why. You are not saving the cold air by persevering. You are spending the compressor.

What a recharge really is, and what it is not

Recharging has a legitimate place. It is just not the place the marketing puts it.

Done properly, on a machine, the sequence is: recover whatever is left in the system, evacuate the loop under vacuum to pull out air and moisture, then weigh in the exact charge the manufacturer specifies. That process is genuinely diagnostic. How much came out tells the technician how much had gone, and how much is missing over a known period is the first real evidence of how big the leak is. A system that has lost a trace over three summers and a system that empties in a fortnight are different problems with different answers, and the recovery step is where that gets measured.

What the recharge does not do is find the leak, and this is where the honest answer to the cost question lives. The charge going back in is the cheap part of the visit. The money is in the diagnosis that comes before it and the repair that comes after.

Look at what that table implies about the question people actually arrive with. Asking what an air conditioning recharge costs is close to asking what a doctor’s appointment costs when the thing you want to know is what the treatment will come to. The recharge is a step in the investigation. If it is being sold to you as the whole answer, the person selling it either has not looked for the leak or has and would rather not discuss what they found.

One reasonable exception deserves stating, because absolutism here would be dishonest. On an older car with a slow loss — cold air that fades gradually over two or three seasons — a properly performed recharge with dye added, followed by driving it until the dye shows up, is a legitimate and economical way to locate a leak that is too slow to find any other way. That is a diagnostic plan with a recharge in it. It is not the same thing as topping the system up every spring and never asking the question.

Three generations of refrigerant, and why two similar cars cost differently

This is the part of the subject a used-car buyer benefits from most, and it is almost never mentioned in the places people go looking for a recharge price.

Vehicle air conditioning has been through a sequence of working fluids, driven by environmental regulation rather than by engineering preference. EPA describes the progression on its motor vehicle air conditioning pages as a transition from CFC-12 to HFC-134a and onward to lower global warming potential refrigerants such as HFO-1234yf. All three are still out there, because cars last a long time and the fleet turns over slowly.

The regulatory backstop is worth stating precisely rather than roughly, because it is easy to turn into a rule of thumb that is not true. Under the Technology Transitions rule EPA issued under the American Innovation and Manufacturing Act, refrigerants with a global warming potential of 150 or greater — which includes HFC-134a — are restricted in newly manufactured and imported light-duty passenger vehicles from Model Year 2025, with medium-duty passenger vehicles, heavy-duty pick-up trucks and complete heavy-duty vans following at Model Year 2028.

What that does not mean is that every car before those model years uses HFC-134a. Manufacturers moved to the newer refrigerant at their own pace, many of them years ahead of any deadline, and the changeover often happened part-way through a model’s life or differed between one variant and another. Two cars of the same age and badge can take different refrigerants. The only reliable way to know is to read the label, which every vehicle carries under the bonnet, stating which refrigerant the system holds.

Read it before you buy, not after. It is a ten-second check that tells you which cost bracket the car’s air conditioning sits in for the rest of the time you own it, and it is the sort of thing our guide to what a pre-purchase inspection covers exists to make routine.

Two consequences of the generations existing side by side are worth carrying with you. The first is that they are not interchangeable and cannot be mixed. Under its Significant New Alternatives Policy Program, established under section 612 of the Clean Air Act, EPA requires each approved refrigerant to be used with its own unique set of fittings, specifically to prevent different refrigerants being accidentally combined. Those fittings appear on the car, on recovery and recycling equipment, on can taps and charging gear, and on the containers themselves — and EPA is explicit that an adapter should not be used to convert one to another. If somebody is reaching for an adapter, they are working around a safeguard that exists to protect the fluid in your car.

The second is that mixed or unidentified refrigerant is a genuine problem rather than a purist’s complaint. A shop that recovers a car’s charge into its machine is putting that charge into equipment shared with every other vehicle it services. Contaminated refrigerant coming out of your car does not stay your problem, which is why shops increasingly identify what is in a system before they connect to it, and why a car whose history includes unknown cans can be quietly difficult to get serviced at all.

Why nobody can just sell you a fix in a can, legally

There is a reason air conditioning work does not behave like fitting a battery or changing an air filter, and it is not the trade protecting itself. It is federal environmental law, and knowing the shape of it makes you a considerably better customer.

Vehicle air conditioning is regulated by EPA under the Clean Air Act rather than by the vehicle safety regulator. Most of the requirements sit in section 609, with sections 608 and 612 playing smaller parts. Three provisions matter to an ordinary owner.

Certification. EPA states plainly that when payment of any kind is involved, including non-monetary consideration such as bartering, any person working on a motor vehicle air conditioning system must be certified under section 609 and must use approved refrigerant handling equipment. The agency also makes clear that this applies regardless of which refrigerant the system uses. So the certification requirement is not a function of the car being old, or the refrigerant being an ozone-depleting one. It attaches to the work.

Equipment. Technicians must use refrigerant handling equipment certified by EPA or by an independent standards testing organisation approved by EPA to certify it. Certified equipment has to be used to remove refrigerant before servicing or repairing a system, or before any other work on the vehicle during which refrigerant could reasonably be expected to escape — front-end accident repair being the obvious everyday example.

Recovery, not release. Section 608 prohibits the intentional release of refrigerant while maintaining, servicing, repairing or disposing of air conditioning and refrigeration equipment, motor vehicle systems included. Recovered refrigerant must be recycled or reclaimed before it goes back into a system — and EPA is specific that this holds even when the refrigerant is being returned to the very car it came out of. Reclamation returns it to virgin specification against the Air-Conditioning, Heating and Refrigeration Institute’s Standard 700; recycling on site removes impurities and oil.

Shops carry paperwork behind all of this. A new shop, or one servicing these systems for the first time, has to certify to its EPA Regional Office that it has acquired and is properly using approved equipment. Records showing that each person using the equipment is trained and certified must be kept on site, along with the name and address of anywhere recovered refrigerant is sent, for three years.

None of that is your responsibility as a customer, and none of it is a reason to be suspicious of a garage. It is context. It explains why the work has a floor under its cost that a shelf product appears to undercut, and it gives you one very easy question to ask, which is whether the shop is section 609 certified. Anybody doing this work properly will answer that instantly and without irritation.

There is one more restriction worth knowing if you own something genuinely old. The sale of refrigerant containing ozone-depleting CFCs or HCFCs is restricted to certified technicians, and CFC-12 in containers of less than 20 pounds may only be sold to technicians certified under section 609, with the seller obliged to verify that certification. If you have a CFC-12 car, the fluid for it is not a retail product.

What each part does, and how each one fails

You do not need to be able to repair any of this. You do need enough of a model to follow a conversation about a quote, and to notice when the story you are being told does not hang together.

The compressor

The pump of the system, belt-driven from the engine, engaged and disengaged by an electromagnetic clutch on the front of it. Pressing the air conditioning button does not directly switch on the compressor; it asks the car’s control system for cooling, and the control system decides. It will refuse if pressures are out of range, which is the mechanism that protects a compressor on an empty system — and which is why a car that has leaked out completely often gives you no cold air and no clutch engagement at all rather than warm air with a running compressor.

Compressors fail in two importantly different ways. The clutch, coil or control circuit can fail while the compressor itself is fine, which is a much smaller job. Or the compressor can fail internally, which on a badly starved system means metal particles distributed around the loop. That second case is why an honest quote for a failed compressor frequently includes replacing the receiver-drier or accumulator and flushing the system: leave the debris in and the new compressor follows the old one. A quote that includes those items is not being padded. A quote that omits them is the one to ask about.

The condenser

Effectively a second radiator for the refrigerant, mounted at the very front of the car so that it gets the coldest, fastest air available. That position is not an accident and neither are its consequences: it is the most exposed heat exchanger on the vehicle, taking stones, grit, salt and insects head-on for its whole life, and it is the first thing damaged in a minor front-end knock. A punctured condenser empties a system in minutes.

Because it shares the airflow at the nose of the car with the engine’s own radiator, the two systems interact, and a car with a debris-packed or damaged cooling pack can present with poor air conditioning and a temperature gauge that misbehaves in slow traffic. Our guide to a temperature gauge that climbs deals with that from the cooling side and is the page to read if the gauge is involved; the point to take from here is only that the two complaints can share a single physical cause, so it is worth mentioning both to whoever looks at the car.

The evaporator

The part that does the actual cooling, and the worst-placed component in the system. It lives inside the heating and ventilation housing behind the dashboard, which is where it has to be, and which means that reaching it is frequently a dashboard-out operation measured in many hours of labour. It is also the part most likely to be quietly corroding, because it spends its life cold and wet.

That combination — likely leak site, extremely expensive access — is why an evaporator leak is the outcome that turns a modest air conditioning repair into a decision about whether to repair the car at all. It is also why a seller who genuinely knows a car has an evaporator leak has a powerful incentive to describe the situation as needing a top-up.

The expansion valve or orifice tube

The metering restriction between the high-pressure side and the low-pressure side, and the point at which the refrigerant is allowed to expand and boil. Small, cheap, and capable of producing symptoms wildly out of proportion to its price. Blocked, it starves the evaporator and you get little or no cooling on a system that has a perfectly adequate charge. Stuck open, it floods it. Both faults present as poor cooling with normal-looking fluid levels, which is precisely why guessing at this system from symptoms alone is a poor strategy and why gauge readings on both sides exist.

The receiver-drier or accumulator

A canister containing desiccant, whose job is to hold the small amount of moisture that inevitably finds its way in. It is a service item rather than a permanent fixture: once a system has been opened to the atmosphere, its desiccant has been used up, which is why proper practice replaces it whenever the loop has been open for a repair. Saturated desiccant lets moisture circulate, and moisture in a refrigerant loop forms acids that attack the system from the inside.

Reading what the car is telling you

Work from the symptom rather than from the part, because the symptom is the information you actually have.

It blows warm all the time

Air is moving through the vents at full force, and it is the temperature of the day. The refrigeration side is not working, and the field of candidates is broad: a system that has leaked out entirely, a compressor or clutch that is not engaging, a blockage at the metering device, a failed pressure sensor, or a simple electrical fault in the control circuit. This is the symptom that leads people straight to a recharge, and it is the one where a recharge most often works and least often lasts.

One observation worth making yourself before you go anywhere: with the engine running and the air conditioning switched on to maximum cooling with the blower high, look at the front of the compressor. The outer pulley turns whenever the engine runs, but the centre plate of the clutch should visibly snap into engagement and turn with it. If it never engages, the car is refusing to run the compressor, and a system that is completely empty is one of the commonest reasons for that refusal. Knowing whether it engages is genuinely useful to whoever you hand the car to.

It blows cold and then goes warm

The most diagnostically interesting of the four, and the one most likely to be missed on a short test drive. Cold at first and warm after ten or twenty minutes usually means the system is working but marginal — and there are two very different reasons for that.

A slightly low charge will cool adequately while the car is moving and the condenser is getting plenty of air, then lose the argument in traffic or on a hot day. Alternatively, the evaporator is icing over: a system with a genuinely low charge or a failed cycling control can drop the evaporator below freezing, at which point the condensation on it turns to a block of ice, airflow through it stops, and the vents go warm. Switch the air conditioning off for a while and it thaws and works again, which is exactly the intermittent, weather-dependent behaviour that makes owners describe the fault as coming and going.

Either way the message is the same one this page opened with. Cold-then-warm is not a system asking for its annual top-up. It is a system operating outside its design range, usually because the charge is not what it should be, and it is telling you so in the only way it can.

No air at all, or hardly any

Take this one out of the air conditioning category entirely, because most of the time it does not belong there.

If very little air comes out of the vents on any setting, hot or cold, the refrigeration circuit is almost certainly irrelevant. The blower is what moves air, and it is a separate system that also feeds your heater and your windscreen demist. Blower motors fail. So do the resistor packs or control modules that set fan speed — a distinctive and very common version of which gives you no air on the lower speeds and full air only on maximum, because the highest setting typically bypasses the resistor. Fuses go. Recirculation and blend-door actuators jam and send air to the wrong outlets, which presents as weak flow from one set of vents while another blows perfectly well.

And, extremely often, the cabin filter is blocked. It is a paper element in the ventilation path, it is cheap, on many cars it lives behind the glovebox and takes minutes to change, and it is one of the most neglected service items on any vehicle. A filter thick with several years of leaves and grit throttles everything downstream of it. People spend real money chasing weak air conditioning on cars where the actual problem is that nobody has opened that hatch since the car was new. Our guide to the maintenance a used car actually needs puts it on the list precisely because it is so easy and so routinely skipped.

The air is cold but it smells

A musty, damp, faintly sour smell for the first minute or two after switching on, fading as the car airs through. The cause is the evaporator, and once you know what the part does the explanation is immediate: it is the coldest surface in the car, warm humid air is blown across it continuously, and water condenses on it exactly as it does on a cold glass. That water is supposed to run off and out through a drain tube onto the road, which is what the puddle under a parked car on a hot day is.

A permanently damp, dark, organically-dusted surface is an excellent place for microbial growth, and that is what you are smelling. Two contributing faults are worth naming. A blocked evaporator drain leaves standing water in the housing rather than on the road, and its other symptom is a wet passenger footwell that gets misdiagnosed as a leaking screen seal. And a cabin filter that has never been changed puts a permanently damp organic mat directly in the airflow.

The prevention is free and slightly counter-intuitive: switch the air conditioning off a few minutes before the end of a journey while leaving the fan running, and the evaporator dries out instead of being parked wet. Smell on its own is not evidence of a refrigeration fault. It is evidence of a wet part doing exactly what a wet part does.

Why the dashboard will not warn you about any of this

Worth saying explicitly, because a lot of buyers use warning lights as a proxy for condition.

Air conditioning faults do not, as a rule, illuminate anything. There is no telltale for a low refrigerant charge on an ordinary car in the way there is for oil pressure or coolant temperature. The system simply declines to produce cold air and leaves you to notice. The check engine light covers emissions-related faults and will not usually be troubled by refrigerant leaving the system, so a car with a clean dashboard and a completely empty air conditioning system is entirely normal — our page on what the check engine light actually means sets out how narrow that lamp’s remit really is.

There is a similar gap in the public data, and since this site normally leans hard on federal records it is worth being straight about it. Vehicle air conditioning is regulated by EPA under the Clean Air Act, as an environmental and refrigerant-handling matter. It is not, in the ordinary case, a vehicle safety matter, so it does not generate the safety recall campaigns and defect complaint records that this site draws on elsewhere. That is a regulatory boundary rather than a comment on how often these systems fail. We have no air conditioning failure data because the agency that collects failure data is not the agency that regulates air conditioning, and we would rather say that than stretch some other category to cover it.

Testing air conditioning on a car you are viewing

Now the part that makes this a used-car page rather than a repair page.

Air conditioning sits in an awkward blind spot at a viewing. It is expensive to put right at the top end and trivial to conceal for the length of a test drive, and for roughly half the year the weather itself supplies a seller with a reason not to demonstrate it that sounds completely reasonable. Most buyers never test it. A good number never think of it until the following June.

Test it on a cold day, and test it properly

The commonest mistake is accepting that air conditioning cannot be assessed in winter. It can. A system does not know what month it is; it removes heat and it dehumidifies, and both are demonstrable at any temperature.

Run the engine until it is properly warm, then set the climate control to its coldest setting with the air conditioning switched on and recirculation off, and the blower at a middling speed rather than maximum. Then look for three things. First, air from the vents that is clearly colder than the outside air — the back of your hand is a perfectly adequate instrument for a difference that should be obvious rather than subtle. Second, the compressor clutch engaging, which on most cars you can both see at the front of the engine and hear as a soft clunk with a small change in idle. Third, and often the most telling in cold weather, the windscreen clearing quickly when you select demist, because that is the air conditioning drying the air whether or not the driver realises it is running.

A car with a manual system where nothing changes when the button is pressed — no clutch, no note change, no difference at the vents — is a car whose air conditioning is not working, in January as much as in July.

Then leave it running

The single most valuable extra step, and the one that catches the fault a short test drive is designed to hide.

Marginal systems work at first. A low charge often produces perfectly convincing cold air for the first several minutes, particularly while the car is moving. Set the air conditioning going at the start of the viewing and leave it on for everything else you do — the paperwork, the tyres, the boot floor, the service history, the whole test drive — then check the vents again at the end. Cold at the beginning and lukewarm forty minutes later is the cold-then-warm fault described above, and it is exactly the fault that a fifteen-minute drive around the block will never surface.

Include some stationary idling in that. Air conditioning that is fine at thirty miles an hour and gives up at a standstill points at condenser airflow — a blocked cooling pack or a fan not running — and standing still is the only condition that shows it.

What a recently recharged system tells you

If a seller volunteers that they have just had the air conditioning gassed up, or you find a receipt for it in the history folder, treat it the way you would treat a brand-new battery on an elderly car: not as bad news, but as a question that has not been answered yet.

Go back to the opening argument. A system that needed recharging was low, and a system that was low had leaked. So the recharge is direct evidence that this car has, or had, a leak. The receipt is not the reassurance it appears to be — it is documentation of a symptom. The question to put, plainly and without any edge to it, is what the leak turned out to be and what was done about it. There are entirely good answers. A seal or an O-ring was replaced, or a condenser was renewed after a stone strike, and there is an invoice showing the part. There are also answers that tell you the leak was never investigated, and those are worth hearing too.

Ask one further question if the answer is vague: has anything ever been added to the system from a can. Sealant products in particular can complicate or prevent later servicing, and it is much better to know before you own the car.

Get it looked at before you commit, not after

If the car is otherwise right and the air conditioning is doubtful, the sequence that protects you is the ordinary one: find out what is wrong before money changes hands. An air conditioning system can be checked on gauges in a few minutes by anybody equipped to work on it, and the answer distinguishes a cheap fault from a very expensive one. That check belongs inside the same visit as the rest of an independent inspection.

It is also worth remembering that the mechanical picture is only half of what you are buying. A car that has had front-end damage repaired is a car whose condenser has been disturbed, and accident history is exactly the sort of thing that lives in records rather than under a bonnet. Running the VIN while you are still standing in the road lets you see what the paperwork says about a car before you agree anything, which is the right order to do it in.

Turning what you found into a price

Suppose the air conditioning does not work and you still want the car. That is a perfectly reasonable position — plenty of good cars have a dead air conditioning system, and on an older one it can be the least important thing about it.

What you should not do is accept an estimate from the person selling it. The range of outcomes here is enormous and the seller has no more idea than you do. An O-ring, a condenser, a compressor and an evaporator span an order of magnitude in cost, and which one applies is a question of diagnosis rather than opinion. A seller who says it is probably just a seal is guessing, and the guess conveniently favours them.

The stronger position is a written diagnosis from somebody independent, brought back to the conversation. That converts an argument about what it might be into a discussion about a document, which is the whole basis of our guide to negotiating the price of a used car. It also tells you something you cannot get any other way: whether this is a car with a cheap fault, or a car whose next owner faces a dashboard-out repair. Those are different cars at the same asking price.

And keep proportion about it. Air conditioning is a comfort system. A car that is sound underneath with dead air conditioning is usually a better buy than a car with ice-cold vents and a history you have not looked at, which is why establishing the recorded past of any car you are serious about comes before the vents do. Fix the order of your questions and the air conditioning takes its proper place: a real cost, worth pricing accurately, and never the thing that decides the purchase.

Common questions

Why is my car air conditioning not blowing cold?

Most often because the refrigerant charge is low, and a low charge means the system has leaked, since nothing consumes refrigerant in normal use. Other common causes are a compressor or clutch that is not engaging, a blocked expansion valve or orifice tube, a failed pressure sensor, or a fault in the control circuit. If the air is not merely warm but barely moving, look first at the blower and the cabin filter rather than the air conditioning at all — that is a ventilation problem wearing an air conditioning costume.

How often should air conditioning be recharged?

On a healthy system, never. It is a sealed loop and the charge that went in at the factory is meant to stay there. The idea of an annual or biennial top-up as routine maintenance has no basis in how the system works. If a car genuinely needs recharging on a schedule, that schedule is a measurement of how fast it is leaking, and the useful piece of information in it is the leak rate rather than the date.

Does a recharge fix the problem?

It restores cold air, which is not the same thing. Performed properly it is also a diagnostic step, because recovering what is left tells the technician how much had escaped and over what period. What it cannot do is find or close the leak. If a recharge is being sold as a complete repair rather than as part of a plan that includes locating the leak, the leak is still there and the charge is on its way out again.

Can I use a top-up can from a shop myself?

We would advise against it, for reasons that go beyond the obvious. You cannot weigh a charge with a can, and overfilling is its own fault. Many such products include sealants that can complicate later professional servicing. Most importantly, EPA has publicly identified a counterfeit product sold in small cans through online retail platforms for motor vehicle air conditioner use, marketed as Cool Penguin “F-12”, that the agency reports consisted of CFC-12, CFC-114, HCFC-142b and HCFC-22 alongside non-ozone depleting components. You cannot verify what is in a can by looking at the listing, and putting an unidentified mixture into a sealed system is a durable problem rather than a temporary one.

Is it bad to keep driving with the air conditioning not working?

Driving the car is fine. Continuing to run the air conditioning while it is not cooling is not. The compressor is lubricated by oil carried around in the refrigerant itself, so a system that has lost part of its charge has usually lost oil too, and running the compressor in that state is running it short of lubrication. Switch it off and leave it off until it has been looked at. That one decision can be the difference between a seal and a compressor.

How can I test air conditioning when I view a car in winter?

Warm the engine, select the coldest setting with the air conditioning on and recirculation off, and check for three things: air noticeably colder than the ambient temperature, the compressor clutch engaging at the front of the engine, and the windscreen clearing quickly on demist, since the system dehumidifies as well as cools. Then leave it running for the whole viewing and check again at the end. Cold at first and lukewarm later is the fault a short drive is least likely to reveal.

What does it mean if a used car has just had its air conditioning recharged?

It means the system was low, and a system that was low had leaked. The receipt documents a symptom rather than a cure. Ask what the leak was found to be and what was done about it, and ask whether anything has ever been put in from a can. Good answers exist and come with an invoice for a part. A shrug means the leak was never investigated, and you are buying it along with the car.