Car Air Conditioning Problems: Symptoms, Causes and When It Is the Compressor

How does car air con work, and where does it go wrong?

Car air con moves heat rather than making cold. Refrigerant picks up heat from the cabin air at the evaporator behind the dashboard, carries it to the front of the car and sheds it through the condenser, then comes back for more. The compressor is the pump that keeps that loop moving — driven by the engine’s auxiliary belt on most cars, and electrically from the high-voltage battery on hybrids and electric cars.

Every fault on this page breaks that loop at one point. Finding the point is the diagnosis.

It is more than a comfort feature. Air con dries the air as it cools it, which is why it clears a misted windscreen so quickly — a failed system makes wet-weather visibility harder to keep on top of.

Car air conditioning problems by symptom

Start from what the car is doing. The third column separates the faults that genuinely are the compressor from the far larger number that only look as if they might be.

Three rows point firmly at the compressor, and the clearest of them — pressures that barely move when the air con is switched on — needs gauges to confirm. Everything else is more common and worth ruling out first.

Low gas and leaks: why a regas comes first

No car system is perfectly sealed. Refrigerant seeps slowly through hose walls and seals, so a car that cooled hard when new can lose its edge with no single fault. That gradual fade is what a regas is for: the old charge is recovered, the system is held under vacuum to prove it is tight, and the manufacturer’s weight of refrigerant and oil goes back in.

Faster loss means a leak. Condensers take stone chips behind the grille, pipe joints and seals dry out, and evaporators corrode out of sight inside the dashboard. Regassing a leaking system buys weeks at best, so the leak test belongs before the recharge.

Low gas matters to the compressor too. Its oil travels with the refrigerant, so a system running short starves the pump; the pressure switch or sensor that stops the compressor engaging on a nearly empty system is there to protect it.

Faults that are not the compressor

Condenser fan. At speed, air is forced through the condenser for free; in traffic an electric fan has to do the work. When that fan stops, the air con cools on the motorway and goes warm in a queue. The condenser shares the front of the car, and often its fan, with the engine radiator, so the same fault can nudge the temperature gauge up — our guide on how to prevent your car from overheating covers that side.

Blocked condenser. Leaves, road dirt and insects packed between condenser and radiator stop heat escaping. Pressures climb, and the system may cut the compressor out to protect itself.

Smells and water. Moisture condenses on the evaporator whenever the air con runs and should drain out under the car. If the drain blocks, water collects in the case, can reach the front footwell, and mould grows on the evaporator — the stale smell as the fan starts. An evaporator clean and a fresh cabin filter deal with it.

Electrical supply. A fuse, relay or failed pressure sensor can stop a healthy compressor ever being switched on, because the climate control will not run the pump if it cannot read system pressure.

Air con compressor failure symptoms

A failing compressor usually shows itself in one of four ways, and which one tells your garage a lot about what else in the system needs attention.

The clutch will not engage

On a belt-driven compressor with a clutch, switching on the air con should bring a click and set the centre plate of the pulley turning. If it never does, the compressor may still be innocent: low pressure, a fuse, a relay or a pressure sensor will all keep the clutch off. It comes into the frame once the charge and the supply are proved good and the clutch still will not pull in — heat and wear eventually defeat the coil and the friction faces.

Many newer compressors have no clutch at all. The pulley turns whenever the engine runs and output is controlled inside the pump, so there is no click to listen for.

Noise with the air con switched on

Grinding, rumbling or knocking that starts with the air con and stops when it is switched off is coming from inside the compressor, usually where lubrication has broken down and moving parts have begun to wear. Switch the air con off and have it looked at: a grinding compressor is shedding metal into the system. A rumble that carries on with the air con off points more to the pulley bearing, which spins with the engine regardless.

Seized

A compressor that locks up either sets the belt squealing or, on a clutched unit, makes the clutch slip and overheat. Clutchless designs usually have a pulley built to break its drive if the pump seizes, so the belt keeps turning everything else. Either way, a seizure generally means debris has already travelled round the system.

Turning, but not pumping

This is the quiet failure, and the one most often mistaken for low gas. The compressor turns, there is no noise and no warning light, yet the air stays warm and the pressures hardly move when the air con is switched on. It is the classic presentation of the Sanden PXE16 fitted across VW Group cars: no change in engine sound, nothing on the dashboard, simply no cold air.

On variable-output compressors, how hard the pump works is set by a control valve that the climate control drives electrically. When that control fails, the pump sits at minimum output and behaves exactly like a system short of gas — which is why these cars are often regassed more than once before anyone measures what the compressor is actually doing.

Why air con compressors fail

A compressor seldom fails without a cause, and the cause decides what else needs doing when it is repaired.

• Lubrication loss. A leak or a long spell on a low charge starves the pump of oil. On some designs the shortfall is built in: the VW Group compressor behind many Golf Mk5 air con faults suffers from a lack of lubrication inside the unit, leading to metal fatigue and, in the end, a broken shaft.
• Contamination. Moisture from a system left open or leaking reacts with the oil to form acids, and metal particles from one failure are carried round to block pipes, the expansion valve and the drier.
• Clutch and bearing wear. The clutch coil runs hot, and the friction faces and pulley bearing wear with every engagement and every mile.
• Control faults. On variable-output units the electrically driven control valve can fail on its own, leaving a mechanically sound pump that no longer responds.
• Weak original designs. Early revisions of the Denso 7SEU17C fitted to Mercedes models failed often enough to be superseded, and the Sanden SD6V12 on BMW Mini models is another known common failure.

Regas or compressor? When a regas will not fix it

A regas answers one question: was the system short of refrigerant? If it was and nothing is leaking, the regas is the fix. If the compressor has stopped pumping it cannot help — the charge will be correct and the vents will still blow warm.

A regas is unlikely to be the answer when:

• the car was regassed recently, has held its charge, and still does not cool;
• the pressures barely change between air con off and on;
• the compressor grinds, knocks or has seized;
• the clutch engages normally, or there is no clutch, and the air stays warm on a correct charge.

The reverse catches people out. A leaking system regassed again and again runs its compressor short of oil between visits, so the quick fix, repeated, can become the reason the compressor fails. Leak first, regas second.

What happens when the compressor is the fault

Your garage removes the compressor and posts it to us. We bench test it to confirm the failure and find what caused it, then rebuild it with genuine OEM components — or, where a superseded design is the better answer, supply a replacement unit instead, since we stock both. Either way it goes back ready for your garage to fit, and our air conditioning repair range lists compressors by type and vehicle.

Refitting after a compressor failure is more than bolting it in, and each step is part of the job rather than an extra. The system is flushed to clear debris, the drier and expansion valve are renewed where contamination has reached them, the correct oil goes in, and the system is evacuated and charged to the manufacturer’s weight. That gives the rebuilt compressor a clean system to work in.

Car air con questions, answered