P0505: Idle Air Control System
What to do
Usually driveable. Expect a high, low, hunting or unstable idle, and possibly stalling as you come to a stop. Before agreeing to a new idle valve or a throttle body clean, ask whether the engine was checked for vacuum leaks — the idle control is very often compensating for one rather than failing itself.
What this code means
The module cannot hold the idle speed it wants. On older engines a dedicated idle air control valve bypasses the closed throttle plate with a metered amount of air; on later drive-by-wire engines the throttle body itself opens fractionally instead, and there is no separate valve at all.
This is a commonly misdiagnosed code, and the misdiagnosis has a specific shape. The instinct is to clean or replace the idle valve, because that is the part the code names. But the idle control system is the thing compensating, not the thing failing — an unmetered air leak means the engine already receives more air than commanded, so the module closes the idle valve as far as it will go and still cannot bring the speed down. The code reports that it ran out of authority, not that the valve broke.
So the useful first question is which direction the control has run out in, and P0506 and P0507 answer it directly. Where only P0505 is stored, read the idle air control position or throttle angle at warm idle: pinned fully closed points at a leak, and pinned fully open points at a restriction, a binding throttle plate, or a genuine mechanical fault. Cleaning the throttle body on an engine with a split intake hose achieves nothing except a few days of delay.
Likely causes
Start here
- Vacuum leak in intake manifold or hoses · moderate
- Carbon-fouled idle air control valve · easy
- Carbon deposits in the throttle body · easy
- Faulty idle air control valve · moderate
Less common
- Failed PCV valve or hose · easy
- Split or disconnected intake boot · easy
- Binding throttle plate or linkage · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P0505.