Air con compressor not engaging: causes and what you can do

You mainly notice it because the air does not get cold while the air con is on. The air con compressor, sometimes called the air con pump, pumps the refrigerant round the system. Without a working compressor the air con does not cool. These signs often come with it:

Please note: many newer cars have a variable displacement compressor without a magnetic clutch. It turns with the engine all the time and controls its output with a valve inside. Electric cars and many hybrids have an electric compressor. On those cars you hear no click, even when everything is working.

In cold weather it is normal for the compressor not to engage. The Škoda Fabia owner’s manual gives an outside temperature above about +2 °C as a condition for cooling. The limit differs by make and model. So check the owner’s manual of your car. If the air con does not cool on a mild or warm day, there is a fault.

The compressor can also stay off on purpose in these situations:

If the compressor does not engage when it is freezing, even when you use the air con to clear misted windows, the car is working as the manufacturer intended. It is not a fault.

The table shows for each cause what you notice and how a technician confirms it. The causes we check first are at the top.

A first clue. The cause only becomes clear from a measurement.

Too little refrigerant. This is the first cause we rule out. The pressure sensor monitors the pressure in the system. If there is too little refrigerant, it measures low pressure and the control unit switches the compressor off. That protects the compressor, because the refrigerant carries the oil that lubricates it. According to the ANWB (the Dutch motoring organisation), an air con that runs dry will break down. If the air con has been refilled before, a leak is likely. We then start with a nitrogen leak test.

Pressure too high. The pressure sensor also switches the compressor off when the pressure gets too high. That happens when the condenser at the front cannot get rid of its heat, for example because of dirt or a radiator fan that does not run. You mainly notice it when the car is stationary. Too much refrigerant, for example after a refill with a DIY can, also causes high pressure.

A faulty pressure sensor. A sensor that sends a wrong value switches the compressor off even though the refrigerant charge is fine. HELLA names a faulty pressure sensor as a common cause of air con failure on certain Mercedes models. HELLA describes comparing the sensor value with the reading on a pressure gauge. That is how we work too.

The magnetic clutch. On a belt-driven compressor, an electromagnet connects the compressor to the pulley. If the coil is faulty, or the gap between the clutch plate and the pulley has become too wide through wear, the clutch does not engage. On many compressors the clutch can be replaced on its own. On others the whole compressor has to be replaced.

Fuse, relay or wiring. A blown fuse or a faulty relay stops the current to the clutch. According to the ANWB, a repair for damaged wiring or a fuse need not be expensive. If the same fuse keeps blowing, there is a short circuit, often in the clutch coil.

The control valve in a variable displacement compressor. On a compressor without a clutch, a valve inside controls how much it pumps. If that valve does not work, the compressor pumps nothing. HELLA describes this on the Volkswagen Sharan: if the high and the low pressure are the same, that points to the control valve. The manufacturer does not allow for the valve to be repaired. The compressor is then replaced, the system is flushed and the drier and the expansion valve are replaced.

In an electric car and in many hybrids, the compressor runs on its own high-voltage electric motor. That means it works independently of the engine, so the car can pre-cool before you set off. In many of these cars the air con system also cools the battery. If the compressor does not work, the car may limit fast charging or power.

Only a technician with the right training and the right tools may work on an electric compressor. The oil must be electrically insulating. Nissens states that a high-voltage compressor needs polyolester oil (POE) and that PAG oil must not be used. The manufacturer decides which oil it is. Our technicians hold a GMTO certificate, the Dutch qualification for working on high-voltage systems. Read more about air con for electric and hybrid cars.

You can check a few things yourself before you call. That helps you describe the problem clearly.

1. Check the conditions. Is it a mild or warm day, well above freezing? Is the A/C button on, the fan not set to 0 and the climate control not set to ECO?
2. Listen for the click. Let the engine idle, with the car in P or neutral and the handbrake on. Switch the air con on and off and listen at the front. Keep your hands, hair and clothing away from belts and fans.
3. Look up the fuse in the owner’s manual. It shows which fuse belongs to the air con. If it has blown, replace it once with a fuse of the same rating. If it blows again, have the car checked.
4. Look at the auxiliary belt. Switch the engine off and check whether the belt is intact and tight. Do you hear a squealing or rattling noise when the air con comes on? Then switch it off and read our article about air con noises.

The cost varies widely. A fuse, a relay or an air con service is a small expense. According to the ANWB (air con problems page, in Dutch), a new compressor is an expensive repair, with costs that can rise to € 1,000. That is why we measure first. That way we do not replace a compressor when the cause is a sensor or too little refrigerant.

1. You call with your registration number and tell us what you notice: no click, a flashing light, cutting out or only when stationary. We tell you in advance what the diagnosis costs.
2. We measure the pressures and read the fault codes. We also measure the voltage at the clutch and check how the pressure sensor and the radiator fan work.
3. You receive a quote in advance. We only carry out repairs once you have agreed. If the compressor really needs replacing, our page on air con compressor replacement explains what is involved.

Have you already received a quote for a new compressor elsewhere? Then you can get a second opinion from us. It costs € 145, including up to one hour of investigation. Is the cause too little refrigerant in a sealed system? Then an air con regas is enough. For other faulty parts, see our page on air con repair. If the air con is not working at all, read car air con not working. Everything about our air con service is on our page about car air conditioning in Rotterdam.

Yes. Air con pump is another name for the air con compressor. The part pressurises the refrigerant and pumps it round the system. Without a working compressor the air con does not cool.

On some cars the A/C light flashes when the control unit detects a fault and switches the compressor off. Your owner’s manual explains what the flashing means on your car. Have the fault codes read, so you know which fault the car has stored.

No, not necessarily. Often the cause is too little refrigerant, a fuse, a relay, the pressure sensor or the magnetic clutch. On many compressors the clutch can be replaced on its own. Replacement is only needed when the measurements show that the compressor itself is not pumping.

Yes. A compressor that does not engage usually causes the engine no trouble. If the compressor rattles or screeches, switch the air con off. A seizing compressor can make the auxiliary belt slip or break.

While you drive, the airflow cools the condenser and the pressure stays within the limit. When the car is stationary, the pressure rises if the radiator fan does not work properly or the condenser is dirty. The pressure sensor then switches the compressor off. Have the fan and the condenser checked.

According to the ANWB, a compressor lasts at least 6 years or 120,000 km with normal use and maintenance. Using the air con regularly helps, in winter too. The refrigerant carries the oil that lubricates the compressor.

Yes. At low outside temperatures the compressor stays off on many cars. The Škoda Fabia owner’s manual gives about +2 °C as the lower limit. Check the owner’s manual of your own car, because the limit differs by make.

• ANWB: Airco onderhoud: dit moet je weten (in Dutch)
• ANWB: Airconditioning: soorten en werking (in Dutch)
• HELLA: Mercedes W168/W210: Air conditioning system failure
• HELLA: Volkswagen Sharan: Air conditioning system is not functioning
• Škoda Auto: Škoda Fabia owner’s manual: Climatic (cooling above +2 °C)
• Nissens: Heat pump systems in electric vehicles: poster (in Dutch)