Car symptom
Rough or unstable idle
A rough or hunting idle points to a misfire, a vacuum leak or idle control - narrow it down by when it happens.
Most likely causes
- Misfire (plugs, coils, injectors)
- Vacuum leak
- Faulty coolant sensor (cold)
- Dirty throttle body / IAC
- Worn engine mounts
Checks to run
- Read OBD codes
- Inspect plugs and coils
- Check for vacuum leaks
Tools you may need
OBD-II scanner
Spark plug socket
Carb cleaner
Confirm before replacing. These are hypotheses to check — never buy parts on a guess. If the fault involves brakes, airbags or fuel under pressure, use a professional.
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FAQ
It depends on the cause. Some causes are cheap and quick; others (like a head gasket or seized caliper) need attention soon. Work through the checks in order and confirm before replacing parts.
Start with the free checks listed here. If they involve high voltage, fuel under pressure, airbags or brakes, let a professional handle it.
The interactive diagnostic asks targeted questions to narrow it down, and every cause must be confirmed with a measurement — never guess.