Checking the air conditioning on a used car properly

The system works with a refrigerant circulating in a closed loop. The compressor pressurises it; in the condenser ahead of the radiator it gives up heat to the outside air and turns liquid; an expansion device drops its pressure; and in the evaporator behind the dashboard it boils again, taking heat out of the cabin air. The receiver-drier absorbs moisture that would otherwise freeze in the system or form acid.

The decisive point for a used car: the compressor’s oil is dissolved in the refrigerant and circulates with it. If the system stands empty because it leaks, or is left switched off for years, the compressor gets no lubrication and its shaft seal dries out. A system that is “only” empty is therefore rarely only empty.

That a system loses a little refrigerant over the years is normal — the hoses are not perfectly impermeable. A recharge after many years is therefore not suspicious. Two recharges in two years is.

Two refrigerants are in circulation: the older R134a and the newer R1234yf, which came in with the move to lower global warming potential in newer type approvals. They are not interchangeable, need different service equipment and different couplings — and they cost markedly different amounts.

Which one the car uses is not a guess. A label in the engine bay names the refrigerant and the charge quantity. That is the detail to photograph at a viewing if you want to estimate what a recharge will cost — not the model year, because manufacturers did not switch at the same moment.

In practice: on two otherwise identical cars the same repair can differ noticeably in cost simply because of the refrigerant. That belongs in the sums before you negotiate.

A hand in front of the vent is no test, because the sensation depends on the outside temperature, the humidity and the temperature of your own hand. A simple thermometer in the centre vent is a test. The procedure is always the same, and it takes ten minutes.

Older systems have a magnetic clutch on the front of the compressor. Switching on produces a click and a brief dip in idle speed. That noise is a useful sign: it says the control system is releasing the compressor at all — which it will not do if system pressure is too low.

Newer cars often use externally controlled compressors with no clutch: they turn all the time and vary their displacement through a valve. There is no click there, and its absence is not a finding. Anyone who does not know that looks in the wrong place.

What counts in both cases are noises that do not belong: a grinding or rumbling that arrives with the switch-on, or a belt squeal as it engages — the latter can be the belt, but it can equally be a stiff compressor.

Humidity condenses at the evaporator. The water runs out through a drain hose under the car. A puddle of clear water after a run with the system on is therefore a good sign: the drain is clear.

If there is none and the cabin smells musty, the drain is probably blocked — by leaves, dirt or a kinked hose. The water then finds another route, and on many cars that route leads into the footwell, often on the passenger side and often exactly where control units sit. Damp mats in dry weather are therefore not a cosmetic issue.

To check, look under the car after the test drive and feel under the mats before you get out again.

A musty smell at switch-on usually comes from the evaporator: it is permanently damp, and micro-organisms settle on its fins. Disinfecting helps but recurs as long as the evaporator does not dry out after shutdown. Switching the cooling off for the last minutes of a journey and leaving the fan running reduces it.

The second reason for weak performance is mundane and still gets overlooked: the cabin filter. When it is clogged, little air arrives, and what does arrive does not feel cold even though the circuit is working perfectly. The filter is one of the cheapest parts on the car and reachable in minutes on many models — worth a look before any verdict on the air conditioning.

The parts of the system differ less in price than in the labour their replacement demands. Condenser and drier sit at the front and are accessible. The compressor is expensive but reachable. The evaporator sits in the heater box behind the dashboard, and on most cars that dashboard has to come out.

So with a leaking system the question is not “what does the part cost” but “where is the leak”. A leak trace with dye answers that, and it is the sensible first expense — not the third recharge.

A compressor destroyed mechanically brings one more rule: its debris spreads through the whole circuit. A clean repair therefore also replaces the drier and the expansion device and flushes the lines. An invoice listing only the compressor is the reason the second compressor does not last either.

In an electric car the refrigerant circuit is rarely there for comfort alone. It often cools the battery as well, and where a heat pump is fitted the same circuit works in reverse in winter and heats the cabin on less electricity than a resistive heater would need.

A faulty system then costs more than comfort: range in winter, and potentially charging power at a rapid charger, because the battery cannot hold its temperature. The check is the same as above; the consequences of a finding are larger.

Three questions are enough, and all three can be answered with paperwork: when was the system last charged, was a leak search done at the same time, and which refrigerant does the label name. A seller who produces the invoice answers all three with one sheet.

If it blows cold and the answers are plausible, the subject is closed. If it blows lukewarm, it is an item with a wide range — from a recharge to a dashboard out on the floor — and that range belongs settled before you commit, not after.