Why is my idle high and the revs hang when I lift off?

Both symptoms point the same way. Air is entering the engine through a route the control unit cannot close, most often a perished vacuum hose, a split crankcase breather or a leaking inlet manifold gasket. When you lift off, the control unit shuts the throttle fully but the leak keeps feeding air, so the revs settle slowly instead of dropping cleanly. A sticking throttle plate produces the same effect and should be checked at the same time.

How do I find a vacuum leak?

The reliable way is a smoke test, where a garage feeds harmless smoke into the sealed intake at low pressure and watches for where it escapes. It takes minutes and finds leaks in places that cannot be seen or heard. Without one, inspect every rubber hose on the intake side, squeeze them to find splits that only open under flex, check the brake servo hose and its one-way valve, and look closely at the crankcase breather pipes where they meet the rocker cover.

Will P0507 fail an MOT?

Yes, and this one has a genuine second route to failure. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times. Beyond that, the vacuum leak usually behind P0507 leans the mixture, and the petrol emissions test checks the lambda value at fast idle as well as the carbon monoxide and hydrocarbon figures at natural idle, so the tailpipe result can fail too.

Is a high idle dangerous?

Usually not, but there is one case that is. If the high idle is caused by an air leak, the car creeps harder in traffic and uses more fuel but behaves predictably. If it is caused by a throttle plate or cable that is physically sticking, the throttle may not return properly when you lift off, and that can get worse rather than better. Check by hand that the throttle snaps shut against its stop; if it does not, stop driving the car.

Powertrain code · standard

P0507Idle speed higher than it should be

Standard definition: Idle Air Control System RPM Higher Than Expected

Last updated 27 July 2026

Quick answer

The engine is idling faster than the control unit wants and closing the throttle no longer brings it down. In the great majority of cases, extra air is getting into the engine somewhere it should not.

How the car works this out

The control unit knows the idle speed it is targeting for the current temperature and load, and it can see how far it has already closed the idle control to try to reach it. P0507 is set when the actual speed remains above the target even with the idle control at its minimum. That combination — throttle closed as far as it will go and the engine still idling fast — has one dominant explanation, which is air entering the engine by a route the control unit does not govern.

What you might notice

Idle noticeably faster than normal, often above 1,000 rpm when warm
Revs hanging up for several seconds when you lift off, then dropping slowly
The car creeping forward more strongly than usual at a standstill in an automatic
A hissing or whistling noise from the engine bay at idle
Poor economy around town
Brakes feeling as though they are working against the engine in traffic

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 vacuum leak — a perished hose, a cracked intake pipe, a failed inlet manifold gasket or a leaking brake servo hoseCommon
How to check: A smoke test of the intake system. It is the single most useful diagnostic for this code and finds leaks that no amount of listening will.
2A split or disconnected crankcase breather hose, or a failed positive crankcase ventilation valveCommon
How to check: Inspect the breather hoses, especially where they meet the rocker cover and the intake. They harden and split with heat and age.
3A sticking throttle plate or a throttle body that needs cleaning and relearningCommon
4A sticking or seized idle air control valve on cars that have onePossible
5A sticking throttle cable or linkage on older cars, holding the plate slightly openPossible
How to check: Operate the throttle by hand at the throttle body and feel whether it snaps fully shut against its stop.
6A coolant temperature sensor reading falsely cold, keeping the engine in its warm-up fast idlePossible
How to check: Check the coolant temperature on live data against reality, and look for P0118 stored alongside.

What to do about it

1Have the intake smoke tested. A high idle that will not come down is a vacuum leak until proved otherwise, and a smoke test finds it quickly.
2Check the crankcase breather hoses yourself. They are visible, they perish predictably, and a split one is both a common cause and a cheap fix.
3Check that the throttle is closing fully, particularly on older cars with a cable, because a throttle that does not return is a safety matter as well as a fault code.
4Look at the coolant temperature reading. An engine that believes it is still cold will hold a fast idle deliberately, and that is a sensor problem rather than an air leak.
5Have the throttle body cleaned and relearned if no leak is found.

Can you keep driving?

Briefly, gently

A high idle is more of a nuisance than a danger in most cases, though it makes the car creep harder in traffic and lengthens stopping distances slightly as the engine works against the brakes. The exception is a throttle that is physically sticking rather than an air leak, which can worsen and is genuinely dangerous. Check that the throttle returns properly before dismissing it.

Will it fail an MOT?

The lamp is a major defect and one of 371,859 recorded failures. The emissions consequence is specific to this code: a vacuum leak leans the mixture, and the petrol emissions test measures the lambda value at fast idle as well as carbon monoxide and hydrocarbons at natural idle. A car drawing unmetered air can fall outside the lambda window even when it appears to run acceptably, so P0507 threatens the emissions element on its own account rather than just through the lamp.

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

Why is my idle high and the revs hang when I lift off?: Both symptoms point the same way. Air is entering the engine through a route the control unit cannot close, most often a perished vacuum hose, a split crankcase breather or a leaking inlet manifold gasket. When you lift off, the control unit shuts the throttle fully but the leak keeps feeding air, so the revs settle slowly instead of dropping cleanly. A sticking throttle plate produces the same effect and should be checked at the same time.
How do I find a vacuum leak?: The reliable way is a smoke test, where a garage feeds harmless smoke into the sealed intake at low pressure and watches for where it escapes. It takes minutes and finds leaks in places that cannot be seen or heard. Without one, inspect every rubber hose on the intake side, squeeze them to find splits that only open under flex, check the brake servo hose and its one-way valve, and look closely at the crankcase breather pipes where they meet the rocker cover.
Will P0507 fail an MOT?: Yes, and this one has a genuine second route to failure. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times. Beyond that, the vacuum leak usually behind P0507 leans the mixture, and the petrol emissions test checks the lambda value at fast idle as well as the carbon monoxide and hydrocarbon figures at natural idle, so the tailpipe result can fail too.
Is a high idle dangerous?: Usually not, but there is one case that is. If the high idle is caused by an air leak, the car creeps harder in traffic and uses more fuel but behaves predictably. If it is caused by a throttle plate or cable that is physically sticking, the throttle may not return properly when you lift off, and that can get worse rather than better. Check by hand that the throttle snaps shut against its stop; if it does not, stop driving the car.

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