An idle stuck above 1000 RPM almost always means a vacuum leak, a stuck throttle plate, or a bad IAC valve. The fix is usually under $80 and an hour of your time.
Unmetered air entering past the throttle raises idle. Cracked intake hoses, bad PCV, or torn intake gaskets are the common culprits.
Carbon buildup keeps the throttle plate slightly open. Cleaning the throttle body and learning idle relearn fixes it.
On older cars without electronic throttle, a stuck IAC valve commands too much bypass air. Replace or clean it.
A sensor stuck reading cold makes the ECU command high idle as if engine is warming up. Code P0117 or P0118.
A glitchy TPS reports throttle slightly open when closed. The ECU then commands more idle. Code P0121-P0123.
Idle goes above 2500 RPM and stays there, the car accelerates when you release the brake, or you cannot stop without standing on the brake. Runaway idle is a serious hazard.
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Most modern cars idle at 600-900 RPM warm. Cold idle can be 1200-1500 briefly. Anything above 1000 warm is a problem.
Cold-start fast idle is normal. The ECU commands higher RPM to warm the cat and engine. It should drop within 2-5 minutes.
Yes, very commonly. Carbon buildup on the plate keeps it slightly open. A 15-minute cleaning usually resolves it.
Long term, yes - higher fuel consumption, more carbon buildup, and stressed engine mounts. Fix within a few weeks.
Yes - AC compressor load triggers an idle bump (about 100-200 RPM). If your idle is high even with AC off, that is the problem.