Does my car have an idle air control valve?

If it has a mechanical throttle cable running from the pedal to the throttle body, probably yes. If the accelerator pedal has no cable and the throttle is opened by a motor — a drive-by-wire or electronic throttle, which covers virtually everything built since the mid-2000s — then no. On those cars the control unit sets idle by opening the electronic throttle a fraction, so a P0505 points at the throttle body, its adaptation or an air leak rather than at a separate valve.

Why does my engine stall at junctions with P0505?

Because the idle control has lost its ability to compensate. When you lift off the throttle the engine needs a precise trickle of air to keep turning, and the control unit adjusts it constantly for the loads from the alternator, the air conditioning and the power steering. Carbon in the throttle body, an air leak or a lost adaptation all break that fine control, and the engine falls below the speed at which it can sustain itself.

Will P0505 fail an MOT?

Yes. The 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 it. There is a second risk on the day: the emissions test needs a stable idle to take its readings, and a car that hunts or stalls may prevent the test being completed, which is a further major failure.

Can cleaning the throttle body fix P0505?

Frequently, provided the adaptation is relearned afterwards. Carbon narrows the small airway the idle depends on, and cleaning it restores the airflow. What catches people out is that the control unit has learned the dirty throttle's position as its reference, so after cleaning it is opening the throttle to a setting that now admits far more air than intended, and the idle can be worse than before. A scan tool running the throttle adaptation procedure finishes the job properly.

Powertrain code · standard

P0505Idle control system fault

Standard definition: Idle Air Control System Malfunction

Last updated 27 July 2026

Quick answer

The engine control unit cannot hold the idle where it wants it. On older cars that points at the idle air control valve; on anything with an electronic throttle there is no such valve, and the throttle body itself is doing the job.

How the car works this out

The control unit sets a target idle speed and adjusts the air bypassing or passing the throttle to hit it, compensating for load from the air conditioning, the alternator and the power steering. It monitors how much correction it is having to apply. P0505 is set when the system can no longer hold the target, or when the correction needed has drifted so far from normal that something is clearly wrong. It is a general idle control code, and on modern cars it very rarely means a faulty valve because there is no valve to fault.

What you might notice

Idle hunting up and down, often rhythmically
Stalling when you lift off the throttle or come to a junction
Idle hanging high for several seconds after you release the pedal
Rough idle that smooths out as soon as revs rise
Stalling when the air conditioning or the electric cooling fan cuts in
Hard starting followed by an immediate stall

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 build-up in the throttle body, restricting the small airway the idle control relies onCommon
How to check: Remove the intake pipe and look at the throttle plate and bore. A hard carbon lip around the edge of the plate is the usual finding.
2A throttle body cleaned or replaced without the adaptation being relearnedCommon
How to check: Run the manufacturer's throttle adaptation procedure with a scan tool. On many cars this alone cures the fault.
3A vacuum or intake leak letting in unmetered air the idle control cannot compensate forCommon
How to check: A smoke test of the intake. It finds in minutes what listening for hisses will not.
4A sticking or failed idle air control valve on older cars that have onePossible
5A faulty positive crankcase ventilation valve or a split breather hose, both of which behave like a vacuum leakPossible
6Low system voltage from a failing battery or alternator, which upsets the idle strategyLess common

What to do about it

1Establish whether your engine has an idle air control valve at all. Anything with a drive-by-wire throttle almost certainly does not, and hunting for a valve that does not exist wastes time and money.
2Have the throttle body cleaned and the adaptation relearned as a single job. Cleaning without relearning frequently leaves the idle worse than before.
3Have the intake smoke tested for leaks. A perished breather hose or a cracked vacuum pipe produces exactly these symptoms and costs very little to put right.
4Check the crankcase breathing system, which is a common and often overlooked source of unmetered air on higher-mileage engines.
5Have the battery and charging system checked if the idle problems appear mainly on cold mornings or with heavy electrical loads.

Can you keep driving?

Briefly, gently

The car will generally get you about, but stalling at junctions is more than an irritation: when the engine stops the steering goes heavy and the brake servo assistance disappears after a press or two of the pedal. Leave more room than usual, and get it fixed rather than developing a habit of blipping the throttle at every roundabout.

Will it fail an MOT?

The lamp alone is a major defect and one of 371,859 recorded failures. P0505 causes a specific practical problem during the test: the emissions check requires the engine to hold a stable natural idle while the readings are taken, and a car that hunts or stalls repeatedly may not give the analyser the steady period it needs. An emissions test that cannot be completed is a major failure in its own right.

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

Does my car have an idle air control valve?: If it has a mechanical throttle cable running from the pedal to the throttle body, probably yes. If the accelerator pedal has no cable and the throttle is opened by a motor — a drive-by-wire or electronic throttle, which covers virtually everything built since the mid-2000s — then no. On those cars the control unit sets idle by opening the electronic throttle a fraction, so a P0505 points at the throttle body, its adaptation or an air leak rather than at a separate valve.
Why does my engine stall at junctions with P0505?: Because the idle control has lost its ability to compensate. When you lift off the throttle the engine needs a precise trickle of air to keep turning, and the control unit adjusts it constantly for the loads from the alternator, the air conditioning and the power steering. Carbon in the throttle body, an air leak or a lost adaptation all break that fine control, and the engine falls below the speed at which it can sustain itself.
Will P0505 fail an MOT?: Yes. The 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 it. There is a second risk on the day: the emissions test needs a stable idle to take its readings, and a car that hunts or stalls may prevent the test being completed, which is a further major failure.
Can cleaning the throttle body fix P0505?: Frequently, provided the adaptation is relearned afterwards. Carbon narrows the small airway the idle depends on, and cleaning it restores the airflow. What catches people out is that the control unit has learned the dirty throttle's position as its reference, so after cleaning it is opening the throttle to a setting that now admits far more air than intended, and the idle can be worse than before. A scan tool running the throttle adaptation procedure finishes the job properly.

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