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Rough Idle: Causes, OBD2 Codes & What to Check

A rough idle means the engine runs unevenly when you're stopped — it shakes, the RPM needle bounces, or it feels like it might stall. It usually points to something disrupting the air, fuel, or spark balance at low RPM.

Is it safe to drive?

A mild rough idle is usually safe to drive on short-term, but it tends to get worse, hurts fuel economy, and can leave you stranded if the engine stalls. Diagnose it sooner rather than later — and if the car is stalling or the check-engine light is flashing, stop driving and get it checked.

What to check first

  • Listen for a hissing sound near the intake — a vacuum leak is one of the most common causes.
  • Note whether it's worse cold, warm, or both.
  • Check for a stored OBD2 code with a scanner — the codes below are the usual suspects.
  • Look for obvious culprits: dirty throttle body, worn spark plugs, or a loose air-intake hose.

Most likely OBD2 codes for rough or unstable idle

These trouble codes most often produce this symptom. Open one for its causes, fixes, and typical repair cost on your vehicle.

Typical repair costs

What the common fixes for this symptom usually run (parts + labor). Get an estimate for your exact vehicle free from the AI Mechanic.

timing belt $500–$1,000 spark plugs $150–$400 ignition coil $150–$400 MAF sensor $200–$400

Related reading: the full walkthrough on the blog .

Rough or unstable idle: FAQ

What causes rough or unstable idle?

A rough idle means the engine runs unevenly when you're stopped — it shakes, the RPM needle bounces, or it feels like it might stall. It usually points to something disrupting the air, fuel, or spark balance at low RPM.

Is it safe to drive with rough or unstable idle?

A mild rough idle is usually safe to drive on short-term, but it tends to get worse, hurts fuel economy, and can leave you stranded if the engine stalls. Diagnose it sooner rather than later — and if the car is stalling or the check-engine light is flashing, stop driving and get it checked.

Which OBD2 codes are linked to rough or unstable idle?

Common codes include P0011 (Intake Cam Over-Advanced (Bank 1, VVT)), P0012 (Intake Cam Over-Retarded (Bank 1, VVT)), P0013 (Exhaust Cam Actuator Circuit Fault (Bank 1)), P0014 (Exhaust Cam Over-Advanced (Bank 1, VVT)), P0016 (Crank/Cam Correlation Fault (Bank 1 Intake)), P0017 (Crank/Cam Correlation (Bank 1 Exhaust)). Read a code's page for its specific causes, fixes, and cost.

Do I need an OBD2 scanner?

For a confirmed diagnosis, yes — a scanner reads the stored trouble code. You can triage many symptoms by behavior first, but the code is what pins down the cause. The free AI Mechanic can interpret your code and symptoms for your exact vehicle.

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