P0505 fault code: idle control system fault
Plain-language definition
Idle Control System
What does it mean?
Idle is the engine speed while the engine is running and the accelerator isn't pressed. To keep the idle steady even when loads such as the air conditioning or the headlights switch on, the engine computer adjusts the amount of air going into the engine. On older petrol cars a separate idle valve (IAC, idle motor) does this job; on cars with an electronic throttle, the throttle itself opens and closes slightly to adjust the idle.
The engine computer constantly compares its target idle speed with the measured engine speed. If the actual speed stays noticeably above or below the target for a set time, or an electrical problem shows up in the idle valve's circuit, it stores P0505.
You may notice the idle speed fluctuating, or sitting higher or lower than it should. The engine may also struggle on a cold start, shake when the air conditioning is switched on and stall when the car is stopped, for example at traffic lights.
How serious is it?
This is a Warning-level code; the problem is usually felt only at idle and at low speed. Since the engine stalling at a stop or while manoeuvring can cause trouble in traffic, it's a good idea to find the cause.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Soot and carbon buildup in the throttle body or idle valve
- An air leak on the intake side (vacuum hose, intake manifold gasket or crankcase ventilation valve)
- A failed idle valve (IAC) or electronic throttle
- A problem with the valve's or throttle body's connector or wiring
- Learned idle values being reset after throttle body cleaning or a battery replacement (on some cars)
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- BMW
- Idle Air Control System
- Ford
- Idle Air Control System Malfunction
Related codes
How does it appear in the 2dakika report?
Idle Control System
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.