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Idle Drops When A/C Is Switched On

A slight dip in revs when the air conditioning starts is normal, but a big drop, rough idle or stalling usually means a dirty throttle body, faulty idle control, vacuum leak or a heavy compressor.

Normal risk

Not usually urgent. Get it checked if the engine stalls.

Short answer · Why does my idle drop when I turn on the A/C?

The compressor adds load to the engine. The engine computer raises the idle to compensate. If the throttle body is dirty, the idle valve is faulty or there is a vacuum leak, the engine cannot compensate well and the revs drop, sometimes until it stalls. A compressor that is seizing adds even more load.

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Safe checks you can do

  1. Compare A/C on and offA small dip then recovery is normal.
  2. Check for warning lightsIdle control codes help.
  3. Listen to the compressorGrinding or loud noise suggests a heavy load.

Questions that narrow it down

  1. Does the engine stall?Idle control or throttle.
  2. Is the idle rough without A/C?Vacuum leak or sensor.
  3. Is the compressor noisy?Excess load.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Dirty throttle bodyPoor idle control.
  2. Faulty idle control valveOlder engines.
  3. Vacuum leakUnmetered air.
  4. Heavy or failing compressorExtra load.
  5. Weak battery or alternatorElectrical load.

Questions about Idle Drops When A/C Is Switched On

Is a small drop normal?

Yes, a brief dip that recovers is normal.

Does cleaning the throttle help?

Often, but some cars need an idle relearn after cleaning.