Engine

P0505 — Idle Control System Malfunction

Severity: medium Drive with care, to a shop

What P0505 means

The engine computer compares actual idle rpm with a target and adjusts it using an idle air control (IAC) valve or an electronic throttle. P0505 sets when idle speed stays well off target and the computer cannot correct it. Typical causes are dirt buildup, unmetered air leaks, or a failing idle actuator.

Symptoms

  • Check Engine light on
  • Idle speed hunting up and down
  • Rough or unstable idle
  • Engine stalls when stopping or coasting
  • Idle too high or too low
  • Surging rpm at idle

Possible causes

  1. Carbon and grime in the throttle body and idle air passages
  2. Vacuum leaks: cracked hoses, intake ducts, or intake manifold gasket
  3. Sticking or failed IAC valve
  4. Wiring faults, corroded connectors, or shorts in the IAC or throttle circuit
  5. Faulty electronic throttle body or missing idle relearn
  6. Rarely, PCM software or internal fault

How to diagnose

  1. Scan for all codes and freeze frame data, and look for related codes such as misfires or sensor faults
  2. Inspect intake ducts, vacuum hoses and fittings for cracks and loose connections
  3. Smoke test the intake, or carefully spray cleaner around suspect spots and watch for rpm changes
  4. Remove the throttle body and check the plate and bore for carbon buildup
  5. Check the IAC or throttle connector and harness: pins, power, ground and control signal
  6. Command the IAC valve with a scan tool and see whether idle responds
  7. Check sensors that affect idle: MAF, coolant temperature, throttle position
  8. If nothing turns up, check for PCM software updates

Common fixes

  • Clean the throttle body and idle passages, then perform an idle relearn
  • Replace damaged vacuum hoses and ducts to fix leaks
  • Replace the IAC valve if it sticks or doesn't respond
  • Repair wiring and connectors
  • Reflash or replace the PCM if confirmed faulty

Questions and answers

What does P0505 mean?

The PCM can't hold the engine at the target idle speed. It's usually a dirty throttle body, a vacuum leak, or a bad IAC valve.

Can I drive with P0505?

Briefly, yes, but the engine may stall at a stop or during slow maneuvers. It's best to fix the cause soon.

Will cleaning the throttle body fix P0505?

Often it does, especially with carbon buildup. Afterward, clear the code and run an idle relearn if your vehicle requires one.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.