Why is my car running rough?

Rough running is almost always a misfire — one or more cylinders aren't firing cleanly. Top causes by frequency: worn or fouled spark plugs (cheapest fix), a failed ignition coil, a vacuum leak somewhere on the intake, a dirty fuel injector, or low compression on one cylinder (more serious). Get the codes scanned — a P0301/P0302/P0303/P0304 tells you exactly which cylinder is the problem.

Get a free AI-ranked diagnosis for your exact year, make and model — with fair repair cost ranges and DIY-vs-shop guidance.