Rough Idle – Causes, Codes and Fixes

Rough or unstable idle is one of the most common drivability complaints. It can be caused by vacuum leaks, dirty throttle bodies, misfires, or idle control problems.

What To Do Next

Scan for codes first. If no codes, check for vacuum leaks (smoke test), clean the throttle body, and inspect PCV hoses.

Practical Tips

Listen for hissing (vacuum leak).

Note whether rough idle happens only when cold or all the time.

Recent work on intake or vacuum lines is a clue.

Common Related Codes

These codes frequently appear with this symptom. Click for causes, cost estimates and repair steps.

Code	Description	Severity
P0300	Random/Multiple Cylinder Misfire Detected	High
P0171	System Too Lean (Bank 1)	Med
P0101	Mass or Volume Air Flow Circuit Range/Performance	Med
P0505	Idle Air Control System Malfunction	Med
P0507	Idle Air Control System RPM Higher Than Expected	Med
P0506	Idle Air Control System RPM Lower Than Expected	Med

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