Checking the air conditioning: does it really cool — or is it going to get expensive?

The quick cold test — a reliable verdict in two minutes

The air conditioning is one of the few components you can test reliably without tools and without a test drive — provided you go about it systematically and don't let yourself be fobbed off with a brief hand held to the middle vent. The test takes less than three minutes and gives you a clear verdict on cooling performance, evenness and compressor function.

Set the system to the coldest temperature, the highest fan speed, and activate recirculation — that way the system only has to cool the already-chilled cabin air further and reaches its maximum performance fastest. Direct the airflow to the centre vents so the air isn't rerouted through the warm footwell or the sun-heated windscreen. Keep the engine running: the compressor is driven off the engine by the belt, and with the engine off no classic system cools while stationary. Wait a minute or two until the system is up to temperature — right after switching on, the first air isn't cold yet, and that's not a defect.

Now comes the actual test: hold your hand one after the other in front of every vent, not just the middle one. The air has to become cold quickly and noticeably at all vents, and do so evenly. As a rough guide, the outlet temperature of a healthy system is well below the outside air — at moderate outside temperatures it feels cool to properly cold, and the difference between outside and outlet air on a healthy system is roughly on the order of 15 to 25 degrees. If you want to know more precisely, stick a simple thermometer into the centre vent for a minute. What matters is not the exact figure, but that it becomes cold quickly and holds the cold.

Watch for three anomalies that immediately warrant suspicion: first, uneven cold — if one side blows cold and the other only lukewarm, that points to a problem with the air flaps or the control system, not necessarily the refrigerant circuit. Second, cold that fades after a short time: if the air first turns cold and then warms up again, the compressor may be cycling off because there is too little refrigerant. Third, an audible click when switching on — that's normally the compressor clutch engaging. If that engagement is absent and the air never turns cold, the compressor isn't working at all.

What's behind "it doesn't cool properly" — the four typical faults

"The A/C doesn't cool" is not a diagnosis but a symptom with very different causes and very different cost consequences. The quick cold test tells you that something is wrong — which of the following four faults is behind it is revealed by the exact pattern of behaviour.

Too little or no refrigerant is by far the most common reason. An air conditioning system is not a hermetically sealed system: over the years refrigerant slowly escapes via seals, hoses and screw connections, entirely without an acute defect. When there isn't enough of it, the system cools only weakly, cycles erratically, or shuts off entirely for safety reasons. Typical pattern: it turns slightly cool for a moment and then fades, or the compressor only briefly kicks in and off again. If the level drops faster than can be explained by normal diffusion, there is a genuine leak — then simply topping up is only a stopgap until the leak is found and sealed.

A faulty compressor is the expensive case. The compressor pressurises the refrigerant and is the heart of the system. It announces itself in two ways: either through noises — a grinding, rattling or metallic grating that occurs when the A/C is switched on and disappears when it's switched off — or by the clutch no longer engaging at all so the air never turns cold. A compressor can also wear internally and spread metal filings through the circuit, which leads to further damage. Careful: a compressor that doesn't engage need not be faulty itself — often the control system shuts it off because the refrigerant pressure is too low. Telling apart "compressor broken" and "compressor won't engage because refrigerant is low" is exactly the point where the layperson's check ends.

A contaminated cabin filter or evaporator doesn't affect the cold, but the air. Moisture condenses on the evaporator, and on this permanently damp surface bacteria and fungi take hold. The result is a musty, mouldy-sour smell that comes out of the vents when you switch on the fan — strongest in the first few seconds. An old, clogged cabin filter makes it worse and also throttles the airflow. This is rarely an expensive fault, but a sign of neglected maintenance — and for sensitive people a real nuisance.

A clogged or damaged condenser sits up front behind the radiator grille and releases the heat into the airflow. It silts up with insects, leaves and road salt, or is damaged by stone chips and then leaks. Symptom: the system cools worse when stationary or at low speed than at pace, because it lacks the cooling airflow. Stone-chip holes in the condenser are also a common, gradual leak.

Air conditioning or automatic climate control — the difference and what it means for the check

During the sales conversation you'll hear both terms, often used interchangeably — technically, though, they mean two different equipment levels, and the difference matters when testing.

A manual air conditioning system cools when you switch it on, using the temperature and fan speed you set by hand. It does not adjust itself: set it to cold and full fan and it blows cold and full until you change something. That's ideal for the check — you get maximum performance directly and unfiltered.

Automatic climate control, by contrast, regulates the cabin temperature on its own to a target value you choose and adjusts cooling, fan and air distribution automatically. Higher spec levels control two or more zones separately, so the driver's and front passenger's sides can be set to different temperatures. That's more comfortable in everyday use, but a trap for the test: if you only set the climate control to 22 degrees and the cabin is already cool, it barely cools — not because it's faulty, but because it has reached its target. So for the test, set the climate control to its minimum (often "LO") and the highest fan speed, and then it works at full cooling power like a manual system.

With multi-zone climate control you additionally check each zone individually: set the driver's and front passenger's sides to clearly different temperatures and feel whether the vents on each side actually deliver air at different temperatures. A flap or an actuator that no longer separates them is a fault in its own right, one that easily slips through in a pure cold assessment. The basic test — does it cool quickly and evenly — remains identical for both types; only the operation differs.

Why an air conditioning system needs servicing — and what that means for a used car

Contrary to what many buyers assume, an air conditioning system is not a maintenance-free component you fit and forget. It has several wear and consumable items that build up over the years — and a used car whose A/C was never serviced carries this backlog with it.

The most important point is the gradual loss of refrigerant. Refrigerant slowly escapes over the years via seals and hose connections, even with a perfectly intact system — roughly reckon on a low single-digit percentage per year. After several years without a top-up, the level drops so far that cooling performance noticeably declines and the compressor runs under-lubricated, because the oil dissolved in the refrigerant escapes along with it. That is precisely why an air conditioning system is serviced on a schedule, not "as needed" — an A/C service every few years with a check, evacuation and proper refill is the norm.

On top of that come two more items: the cabin filter, which needs replacing regularly like an oil filter and, if neglected, encourages odour and germs, and the occasional disinfection of the evaporator against the contamination that causes the musty smell. If this work is entirely missing from the service booklet, that's no reason to walk away — but you should assume that an A/C service and probably a filter change are coming up soon, and factor that into your price thinking.

For you as a buyer, the main takeaway is: a weak A/C is not automatically a major defect. Often it's simply an overdue A/C service behind it. It's exactly this uncertainty that accounts for the range in costs, which the next section is about.

From a top-up to a compressor swap — the cost range

No other A/C aspect shapes the negotiation as strongly as the question of what "doesn't cool" ultimately costs. And the honest answer is: the range is enormous — from a small two-figure to a high four-figure sum, depending on which of the faults described above is present. That's why it pays to match the symptoms to the likely cause and rough cost bracket, instead of slapping a blanket "broken" onto the price.

What matters is the catch at the lower and upper end: a plain top-up is cheap — but only worthwhile if there is no genuine leak. Top it up without searching for the leak and the money is gone again within months. Conversely, the compressor swap is the expensive case, often in the mid to high three-figure range, and with ancillary work into the four-figure range too, because with a compressor failure the drier and sometimes lines often have to be replaced as well and the system flushed at some effort. Where your specific case lands within this range can only be stated responsibly after a diagnosis with a refrigerant pressure test and leak search — not by holding a hand up.

Because this diagnosis decides between a two-figure and a four-figure repair bill, the A/C is a classic case for a reliable cost estimate. With the Premium Check from checkdenwagen, the A/C findings feed into the repair-cost calculation, which gives you a well-founded order of magnitude — the strongest basis for pricing an A/C fault cleanly into the purchase price, instead of guessing.

The sales conversation — and the winter problem

The air conditioning has a quirk no other component shares in this form: in winter it can only be tested to a limited extent. At low outside temperatures it cools more weakly by design, because the temperature difference to the surroundings is small and some systems won't even enable the compressor in the cold. A lukewarm impression in January is therefore not proof of a fault — and that's exactly what some sellers use as a convenient excuse.

Don't let that stop you from switching the system on during the viewing anyway. Even in winter you can judge three things that have nothing to do with the outside temperature: whether the compressor audibly engages when switched on (the click of the clutch), whether noises occur that suggest a wearing compressor, and whether it smells musty — contamination smells at any temperature. If the compressor kicks in and runs without any side noises, that's already a good sign. If the air becomes noticeably cooler after a few minutes than the already-cold surroundings, all the better.

In the conversation, ask specifically about the last A/C service and have the receipt shown to you. A documented service a few years ago, plus a clean cold test, makes the A/C an uncritical point. If receipts are missing and it also smells musty or cools weakly, you bring that in as a negotiating point — not with a made-up figure, but with the note that an A/C service or a diagnosis is due.

Where the cold test hits its limits — namely on the question of whether "cools weakly" is behind only missing refrigerant or a beginning compressor failure — is where the inspector's work begins. The A/C is just one point of over 100: how to systematically work through engine, chassis and electronics on the test drive is shown by this magazine's test-drive checklist, and what you can spot on the bodywork, rust and accident traces with the naked eye is covered by the overview article on spotting defects. An on-site inspection from checkdenwagen bundles all of that: an expert comes straight to the seller in approx. 1.5 hours, checks over 100 points including cooling performance and cabin air, and delivers the report within 24 hours — the Standard Check from €289 incl. VAT and travel, the Premium Check from €339 incl. VAT and travel with an additional repair-cost calculation.