What is a normal idle speed?

Most modern petrol engines idle at somewhere between about 600 and 900 rpm once warm, and most diesels a little higher, around 750 to 900 rpm. All of them idle faster when cold, sometimes well over 1,000 rpm for the first minute or two, and they raise the idle when the air conditioning or a heavy electrical load switches on. What matters for P0506 is not the absolute figure but whether the engine is meeting the target the control unit has set for the conditions.

Can a seized air conditioning compressor cause P0506?

Yes, and it is worth checking because it is easy to miss. Everything driven by the auxiliary belt takes power from the engine, and at idle there is very little power spare. A compressor, alternator, water pump or tensioner bearing that is starting to seize adds drag the idle control cannot overcome, so the idle drops and the code sets. Briefly removing the auxiliary belt and seeing whether the idle recovers is a quick way to confirm it.

Will P0506 fail an MOT?

Yes. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against that defect. A low or unstable idle also makes the emissions test harder to complete, because the analyser needs a steady natural idle to take its readings, and a test that cannot be completed counts as a major failure.

Is P0506 the same as a vacuum leak?

No, and it is usually the opposite. A vacuum leak lets extra unmetered air into the engine, which tends to raise the idle and sets P0507 rather than P0506. P0506 means the engine is not getting enough air at idle or is being held back by drag, so the likely causes are a carbon-clogged throttle body, a restricted intake or exhaust, or a belt-driven component beginning to seize.

Powertrain code · standard

P0506Idle speed lower than it should be

Standard definition: Idle Air Control System RPM Lower Than Expected

Last updated 27 July 2026

Quick answer

The engine is idling below the speed the control unit is asking for and the unit cannot bring it up. Something is either restricting the air the engine needs at idle or adding a load it cannot overcome.

How the car works this out

The control unit compares the actual idle speed against its target, which varies with coolant temperature and electrical load. P0506 is set when the actual speed sits below the target by more than an allowed margin despite the idle control being commanded to open up. It is a directional code, and the direction is the useful part: too little air is reaching the engine at idle, or something is dragging on the engine that the idle control cannot compensate for.

What you might notice

Idle noticeably lower and rougher than normal
Stalling at junctions, when reversing, or when the air conditioning switches on
Vibration through the seat and steering wheel at a standstill
Engine catching and then dying immediately after starting
Headlights dimming at idle as the alternator struggles at low speed

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.

1Carbon deposits in the throttle body narrowing the idle airwayCommon
How to check: Inspect the throttle plate and bore. Carbon accumulates around the edge of the plate and is exactly where the idle air passes.
2A restricted air filter or blocked intake ductingPossible
3A sticking idle air control valve on cars fitted with onePossible
4Excessive mechanical drag, such as a seizing alternator, water pump, air conditioning compressor or belt tensionerPossible
How to check: Remove the auxiliary belt briefly and see whether the idle recovers. If it does, the drag is on one of the belt-driven components.
5A heavily loaded diesel particulate filter or a restricted exhaust raising back pressurePossible
6Weak fuelling from a dirty injector, low fuel pressure or a partly blocked fuel filterLess common
7Poor engine condition — low compression on one cylinder, or valve clearances out of specificationLess common
How to check: A compression or cylinder leakage test. This is the last resort, but it is the answer when everything else has been replaced.

What to do about it

1Have the throttle body cleaned and the adaptation relearned. On a petrol engine with any mileage on it, this is the most likely single answer.
2Check the air filter and the intake ducting for restriction while the throttle is being looked at.
3Listen for a bearing noise from the auxiliary belt components. A seizing air conditioning compressor or alternator drags the idle down and gives itself away with a rumble or squeal.
4Have the particulate filter loading read on a diesel, since a heavily blocked filter raises back pressure enough to pull the idle down.
5Ask for a compression test only if the simpler causes are eliminated, and treat a low reading on one cylinder as a separate and more serious conversation.

Can you keep driving?

Briefly, gently

Driveable, with the caveat that a stall at the wrong moment matters more than the code does. Losing the engine as you pull away at a junction leaves you with heavy steering and unassisted brakes for a few seconds. If it stalls regularly rather than occasionally, treat it as urgent.

Will it fail an MOT?

The lamp is the automatic major defect here, one of 371,859 recorded failures. The specific difficulty with a low idle is the emissions test itself: the analyser takes readings at a stable natural idle, and an engine that is idling below its designed speed or stalling repeatedly may not hold steady long enough. An emissions test that cannot be completed is recorded as a major failure just as a failed one would be.

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

What is a normal idle speed?: Most modern petrol engines idle at somewhere between about 600 and 900 rpm once warm, and most diesels a little higher, around 750 to 900 rpm. All of them idle faster when cold, sometimes well over 1,000 rpm for the first minute or two, and they raise the idle when the air conditioning or a heavy electrical load switches on. What matters for P0506 is not the absolute figure but whether the engine is meeting the target the control unit has set for the conditions.
Can a seized air conditioning compressor cause P0506?: Yes, and it is worth checking because it is easy to miss. Everything driven by the auxiliary belt takes power from the engine, and at idle there is very little power spare. A compressor, alternator, water pump or tensioner bearing that is starting to seize adds drag the idle control cannot overcome, so the idle drops and the code sets. Briefly removing the auxiliary belt and seeing whether the idle recovers is a quick way to confirm it.
Will P0506 fail an MOT?: Yes. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against that defect. A low or unstable idle also makes the emissions test harder to complete, because the analyser needs a steady natural idle to take its readings, and a test that cannot be completed counts as a major failure.
Is P0506 the same as a vacuum leak?: No, and it is usually the opposite. A vacuum leak lets extra unmetered air into the engine, which tends to raise the idle and sets P0507 rather than P0506. P0506 means the engine is not getting enough air at idle or is being held back by drag, so the likely causes are a carbon-clogged throttle body, a restricted intake or exhaust, or a belt-driven component beginning to seize.

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