Rough Idle: Top Causes Ranked by Likelihood, OBD2 Codes, and Repair Cost Guide
What Causes a Rough Idle
A rough idle means the engine runs unevenly when the vehicle is stopped or at low RPM — you feel a stumble, shake, or RPM fluctuation rather than smooth consistent rotation. The most common causes: worn or fouled spark plugs that misfire on one or more cylinders during low-load idle conditions. A dirty or faulty mass airflow (MAF) sensor sending incorrect airflow data to the ECU. A vacuum leak admitting unmetered air and leaning out the fuel mixture. A faulty idle air control (IAC) valve unable to maintain stable idle RPM. Clogged fuel injectors causing cylinder-specific lean conditions. A dirty throttle body disrupting airflow at idle.
Diagnosis and Repair Cost
An OBD2 scan is the first step — P0300 or P030X codes point to the misfiring cylinder. Spark plug replacement costs $20 to $60 DIY, $100 to $250 shop. MAF sensor: $50 to $200 DIY, $200 to $450 shop. Vacuum leak diagnosis requires a smoke test or careful visual inspection of intake hoses and gaskets — repair cost varies by location from $20 to $300. IAC valve: $25 to $80 DIY, $100 to $300 shop. Throttle body cleaning: $15 to $30 DIY, $80 to $150 shop.