OBD Code P0505: Idle Air Control System Malfunction — Clean First

What does code P0505 mean?

Code P0505 (Idle Air Control System Malfunction) means the idle speed control system is not maintaining the target idle RPM. On older vehicles this uses a dedicated IAC valve; on newer vehicles with electronic throttle, the throttle plate itself controls idle air.

Symptoms

• Check engine light
• Hunting/fluctuating idle (RPM bounces between 600-1100)
• Stalling at junctions or when coming to a stop
• Idle too high (>1200 RPM) or too low (<600 RPM)
• Hard restart after the engine has been running

Causes & fixes

1. Dirty throttle body (50%)

Carbon and oil deposits prevent the throttle plate from fully closing, or block the idle bypass port.

👉 Throttle body cleaner on Amazon UK — £8-12

Cleaning procedure (15 min):

1. Cold engine, remove the air intake hose from the throttle body
2. Spray generously into the bore while manually opening the throttle
3. Wipe out deposits with a cloth
4. Refit, clear the code, let ECU relearn idle (drive 20 minutes normally)

2. Failed IAC valve (20%)

The IAC valve (step motor) can stick or fail electrically. Test resistance across each coil winding: should be 40-80Ω.

👉 IAC valve on Amazon UK — £15-60

3. Intake air leak

Any air bypassing the throttle confuses idle control. Inspect all hoses and gaskets at idle.

Repair cost

ECU idle relearn

After cleaning the throttle body, some ECUs need to relearn the idle position: leave the engine idling for 10 minutes without touching the accelerator. On Honda and Nissan a specific dealer procedure may be needed.