Rough Idle: Causes, Likely Codes & What to Do
TL;DR
Rough idle = engine shakes/runs unevenly when stopped. Severity: medium. Most likely codes: P0171/P0174 (lean), P0300 (misfire), P0101 (MAF). Top causes: vacuum leak, dirty MAF, worn plugs. Usually drivable, but diagnose to avoid stalling.
What "rough idle" means
At idle the engine must maintain a steady low RPM with a precise air-fuel mix. Anything that upsets that balance — extra unmetered air, a misreading sensor, a weak cylinder, or carbon on the valves — shows up most at idle, where there is little airflow to mask the problem.
Quick diagnosis: IF → THEN
| If… | Then… |
|---|---|
| idle is rough and you have a lean code (P0171/P0174) | suspect a vacuum leak or dirty MAF sensor |
| idle shakes and you have a misfire code | suspect spark plugs or an ignition coil |
| idle is rough only when cold and improves when warm | suspect carbon buildup or a sensor reading off when cold |
| idle surges up and down | suspect a vacuum leak or idle-air-control / throttle-body issue |
Most likely fault codes
| Code | Likelihood | Notes |
|---|---|---|
| P0171 — System too lean (Bank 1) | 35% | |
| P0300 — Random/multiple misfire | 30% | |
| P0101 — MAF sensor range/performance | 20% | |
| P0174 — System too lean (Bank 2) | 15% |
Common causes
- Vacuum / intake leak
- Dirty mass air flow (MAF) sensor
- Worn spark plugs or coils
- Carbon buildup on intake valves / throttle body
- Faulty idle air control or PCV valve
What to do
- Scan for trouble codes — lean and misfire codes point in different directions.
- Inspect for vacuum leaks (hoses, intake, PCV).
- Clean the MAF sensor with MAF-specific cleaner.
- Check spark plugs if a misfire code is present.
- Clean the throttle body / check the PCV valve.
When is it urgent?
Rough idle is usually not an emergency, but if the engine stalls or nearly stalls in traffic, diagnose it promptly for safety.
Frequently asked questions
Why does my car idle rough but drive fine?
At idle there is little airflow, so small problems like a vacuum leak or dirty MAF show up clearly. At speed, more airflow masks them.
Can a dirty MAF sensor cause rough idle?
Yes. A contaminated MAF misreads incoming air, upsetting the idle mixture and often setting a lean code. Cleaning it frequently helps.
Is a rough idle dangerous?
Usually not immediately, but it can worsen into stalling and signals an underlying fault worth fixing.