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P0505 — Idle Air Control System Malfunction

Moderate

Quick answer

P0505 means the idle control system isn’t responding correctly to the computer’s commands — most commonly a carbon-fouled throttle body or idle air control valve. The fix is often just a thorough cleaning.

What it means

The computer holds idle speed by metering a small stream of air (through an idle air control valve on older engines, or by cracking the electronic throttle plate on newer ones) and adjusting timing. These codes set when commanded idle and actual idle disagree beyond tolerance.

Years of crankcase vapors leave carbon deposits exactly where idle air flows. That single fact explains why the most common repair for the whole idle-code family is cleaning, not parts.

Common causes

Ordered from most to least likely.

  1. 1.

    Carbon-fouled throttle body

    Carbon around the throttle plate changes idle airflow — and cleaning it is free-ish.

  2. 2.

    Vacuum leak

    Extra unmetered air raises idle; the computer may not be able to compensate.

  3. 3.

    Failed or sticking idle air control valve (cable-throttle engines)

    The IAC meters idle air; carbon jams them.

  4. 4.

    Electronic throttle body fault (drive-by-wire engines)

    Worn motor or position sensors inside the unit.

  5. 5.

    PCV valve stuck

    A stuck-open PCV behaves like a vacuum leak.

How to diagnose it, step by step

Cheapest and most likely checks first.

  1. 1 Look at idle behavior cold vs. warm

    Note actual vs. desired RPM on the scanner. Surging, hunting, or stalling patterns help separate air leaks from control faults.

  2. 2 Inspect for vacuum leaks

    Especially for high-idle codes: spray-test around the intake and check the PCV system.

  3. 3 Clean the throttle body (and IAC if equipped)

    Remove the intake boot and clean the plate, bore, and idle passages with throttle body cleaner. On many engines this alone resolves the code — some drive-by-wire vehicles need an idle relearn afterward.

  4. 4 Test the IAC or throttle body electrically

    Measure the IAC solenoid/stepper per spec, or watch the electronic throttle’s commanded vs. actual position on the scanner.

Parts & tools you may need

  • OBD-II scanner (code reader with freeze frame / live data)
  • Throttle body cleaner and a soft brush
  • Carb/brake cleaner (vacuum leak testing)
  • IAC valve or throttle body (only if cleaning and tests fail)
  • New throttle body gasket

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Related codes

Frequently asked questions

Can I drive with P0505?
Generally yes, with annoyances: stalls at stoplights or a racing idle. A stalling engine loses power steering and brake assist, so treat frequent stalling as urgent.
Will cleaning the throttle body really fix it?
It’s the highest-probability cheap fix for the whole idle family. Do it before replacing anything, and perform the idle relearn procedure if your vehicle needs one.
Why is it worse with the AC on?
The AC compressor adds load at idle, which the idle control must compensate for. A marginal system fails first under that extra load.