Will P0645 fail an MOT even though air conditioning is not tested?

Yes, and it is a frustrating but straightforward rule. Air conditioning is not part of the MOT and no tester will check whether it blows cold. But P0645 illuminates the engine management lamp, and an illuminated malfunction indicator lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times in the DVSA data behind this site. The tester records the lamp, not the reason for it.

Is it safe to drive with P0645?

Yes, in almost every case. The circuit controls the air conditioning compressor clutch and nothing to do with how the car drives, steers or brakes. The one thing to check is that the compressor pulley still spins freely, because a seized compressor puts the auxiliary belt at risk, and losing that belt means losing the alternator, water pump and power steering assistance at the same moment.

Why does my air conditioning not work with P0645?

Because the circuit that engages the compressor is faulty, so the compressor never starts pumping. When the air conditioning is switched on, the control unit energises a relay which pulls in an electromagnetic clutch on the compressor pulley. With a fault in that circuit the clutch never engages, the compressor freewheels, and the air stays warm. You would normally hear a distinct click from the engine bay when the system engages, and its absence is a useful clue.

Can I just ignore an air conditioning fault code?

You can live with it comfortably enough until MOT time, and then it will cost you a failure certificate. Because the code lights the engine management lamp, it is treated exactly like a serious emissions fault by the test. If the car is due soon, it is worth getting the relay or the wiring sorted, and it is usually a modest job compared with most things that light that lamp.

Powertrain code · standard

P0645Air conditioning clutch relay circuit fault

Standard definition: A and C Clutch Relay Control Circuit

Last updated 27 July 2026

Quick answer

The engine control unit has found a fault in the circuit that engages the air conditioning compressor. The air conditioning will usually stop working, and the engine light comes on for what is otherwise a comfort feature.

How the car works this out

When you switch the air conditioning on, the engine control unit does not power the compressor directly. It energises a relay, which engages the compressor's electromagnetic clutch, and at the same time the unit raises the idle slightly to cope with the extra load. It monitors that relay circuit for the expected electrical response. P0645 is set when the circuit is open, shorted to earth or shorted to power. Notably, the fault is in the control circuit — the compressor, the refrigerant charge and the condenser are all outside what this code describes.

What you might notice

Air conditioning blowing warm and never getting cold
No click from the engine bay when the air conditioning is switched on
Engine management lamp on with no change in how the car drives
On some cars, the demister taking much longer to clear the windscreen
Idle no longer rising when the air conditioning is selected

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A failed air conditioning clutch relayCommon
How to check: Swap it with an identical relay elsewhere in the fuse box and see whether the fault follows. It is the cheapest test available and relays fail often.
2A failed compressor clutch coil, which has gone open circuit or shortedCommon
How to check: Measure the coil resistance at the compressor connector against the manufacturer's figure.
3Damaged or corroded wiring to the compressor, which sits low at the front of the engine where road spray and salt reach itCommon
4A corroded or heat-damaged relay socketPossible
5A seized compressor drawing excessive current, or one that has been electrically disconnected after a previous failurePossible
How to check: Check whether the compressor pulley turns freely and whether the clutch plate can be pulled in by hand with power applied.
6A fault in the control unit's driver for that circuit, which is uncommonLess common

What to do about it

1Swap the relay first. It costs nothing to try and it resolves a good share of these.
2Check the wiring at the compressor. The connector is low down at the front of the engine and is one of the more exposed plugs on the car.
3Have the compressor clutch coil measured before assuming the compressor has failed. The coil is sometimes available separately, which turns a large job into a modest one.
4Ask whether the circuit has been disconnected deliberately. A compressor that seized in the past is sometimes isolated to stop it destroying the belt, and the code is the leftover evidence rather than a new fault.
5Get the lamp cleared before the MOT even though the fault only affects comfort. The tester does not distinguish between a serious code and a trivial one.

Can you keep driving?

Yes

Entirely. Nothing about the way the car drives, steers or stops is affected, and the only loss is cold air and a slower windscreen demist. The single exception is a compressor that has seized mechanically: if the pulley is not turning freely, the auxiliary belt is at risk, and a shredded belt takes the alternator, water pump and power steering pump with it.

Will it fail an MOT?

This is the clearest example anywhere of a lamp mattering more than the fault behind it. Air conditioning is not tested during an MOT at all, so nothing about the compressor, the refrigerant or the cold air is inspected. What matters is that the engine malfunction indicator lamp is illuminated, and that is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures. A car that is otherwise perfect will fail on a relay serving a comfort feature.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Will P0645 fail an MOT even though air conditioning is not tested?: Yes, and it is a frustrating but straightforward rule. Air conditioning is not part of the MOT and no tester will check whether it blows cold. But P0645 illuminates the engine management lamp, and an illuminated malfunction indicator lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times in the DVSA data behind this site. The tester records the lamp, not the reason for it.
Is it safe to drive with P0645?: Yes, in almost every case. The circuit controls the air conditioning compressor clutch and nothing to do with how the car drives, steers or brakes. The one thing to check is that the compressor pulley still spins freely, because a seized compressor puts the auxiliary belt at risk, and losing that belt means losing the alternator, water pump and power steering assistance at the same moment.
Why does my air conditioning not work with P0645?: Because the circuit that engages the compressor is faulty, so the compressor never starts pumping. When the air conditioning is switched on, the control unit energises a relay which pulls in an electromagnetic clutch on the compressor pulley. With a fault in that circuit the clutch never engages, the compressor freewheels, and the air stays warm. You would normally hear a distinct click from the engine bay when the system engages, and its absence is a useful clue.
Can I just ignore an air conditioning fault code?: You can live with it comfortably enough until MOT time, and then it will cost you a failure certificate. Because the code lights the engine management lamp, it is treated exactly like a serious emissions fault by the test. If the car is due soon, it is worth getting the relay or the wiring sorted, and it is usually a modest job compared with most things that light that lamp.

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