Parts
$10–150
P0505 — Idle Control System Malfunction
CAUTIONIs it safe to drive? Idle may be erratic or the engine may stall — drivable but stalling at intersections is a hazard; don't put off the diagnosis.
What P0505 means
The ECM monitors actual idle speed against its target and sets P0505 when it can't maintain a stable idle through the idle air control system. On modern drive-by-wire engines the throttle body motor handles idle; on older engines a separate IAC valve controls it.
Most likely causes (in order)
- Throttle body dirty with carbon restricting idle air (clean it first — free fix)
- Idle air control (IAC) valve clogged, stuck, or burned out
- Vacuum leak (any unmetered air raises idle unpredictably)
- Throttle position sensor worn (affects target idle calculation)
- PCV valve stuck open dumping excessive crankcase vapors at idle
Symptoms you might notice
- erratic idle
- idle hunts up and down
- stalling at stops
- rough idle when AC turns on
What to check first
Clean the throttle body and IAC passage with throttle body cleaner before replacing anything — carbon buildup is the cause in probably half of P0505 cases and cleaning takes 20 minutes. Spray, scrub with a toothbrush, reinstall, and reset idle relearn procedure per your specific vehicle.
Repair cost & difficulty
Labor
0.5–2 hr
Difficulty
DIY-friendly