AutoDiag

P0505

Idle Air Control System

Moderate· Fix soonDIY friendlyPowertrainGeneric, all makes

What it means

The ECU cannot control the idle speed. On older engines this is the idle air control (IAC) valve; on drive-by-wire engines the throttle body controls idle directly.

Is it safe to drive?

Usually drivable, but the engine may stall when you stop or idle very high.

Symptoms

  • Idle too high or too low
  • Stalling when coming to a stop
  • Surging idle

Probable causes

Most likely first

  1. 1Carbon-clogged idle air control valve or throttle body
  2. 2Failed IAC valve
  3. 3Vacuum leak
  4. 4Wiring fault

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Clean the throttle body and IAC passages with throttle body cleaner.
  3. 3Check for vacuum leaks.
  4. 4Command idle changes with a scan tool and see if RPM follows.
  5. 5Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Clean the throttle body and IAC valve
  • Replace the IAC valve
  • Perform an idle relearn

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.