If your engine is shaking, surging, or vibrating at a stop, that's a rough idle - and it almost always triggers an OBD2 code before it gets worse. Plug in a scanner and you'll likely see one of the codes below. If you already have your code, enter it on the diagnosis page and get your first cause free.
When one or more cylinders misfire, the engine loses its rhythm. You'll feel it as a shake or stumble, especially at idle when nothing masks the vibration.
View Full Diagnosis - P0300 →A single-cylinder misfire means one specific cylinder isn't firing correctly. Common causes are a bad spark plug, a failing ignition coil, or a clogged fuel injector on that cylinder.
View Full Diagnosis - P0301 →A lean condition means too much air and not enough fuel in the mix. The engine struggles to hold a stable idle because combustion is inconsistent.
View Full Diagnosis - P0171 →Same as P0171 but on the opposite bank. Often appears alongside P0171, pointing to a vacuum leak or a failing mass airflow sensor.
View Full Diagnosis - P0174 →Enter it below for a free diagnosis. You'll get the most likely cause instantly - no account needed.
Get Free DiagnosisDon't have a scanner? Most AutoZone and O'Reilly locations read codes for free.
If your scan tool is showing one of these codes alongside rough idle, that's your starting point. Click any code for the full diagnosis, common causes, and repair costs.
It depends on severity. Mild rough idle at a stop that smooths out while driving is usually safe for short distances, but diagnose it soon. If the idle is severe or the check engine light is flashing, stop driving - a flashing light means an active misfire that can damage your catalytic converter.
This is a classic sign of a vacuum leak or failing idle air control valve. When cold, the engine runs at higher idle to compensate. Once it warms up and drops to normal idle speed, a small leak becomes noticeable. Check for codes P0171, P0174, and P0505.
Spark plugs and ignition coils are the most common cause, especially on cars over 80,000 miles. If you haven't replaced your plugs recently, that's the first thing to check after pulling codes. P0300 or P0301-P0306 almost always means ignition system attention is needed.
It depends on what the code points to. Spark plugs are $20-80 in parts if you do it yourself. A vacuum leak can be under $10 to fix yourself or a few hundred at a shop. An ignition coil is typically $30-80 per coil. Pull your code first - that tells you exactly where to look.