Why is my car running rough or idling rough?

Rough idle and rough running are nearly always a misfire — one or more cylinders aren't combusting properly. The top causes are worn spark plugs, failed ignition coils, vacuum leaks, dirty fuel injectors, or a dirty throttle body. A scan tool reading P0300–P0308 will pinpoint the cylinder.

Cost range: $30–$1,200. Severity: medium. Safe to drive? Solid CEL: short distances OK. Flashing CEL: stop driving — raw fuel destroys the catalytic converter.

Most likely causes

1. Worn spark plugs — Most plugs need replacement at 60–100k miles (iridium can go further). Cheap, easy DIY on most cars. ($30–$200, common)
2. Failed ignition coil — Causes a single-cylinder misfire (P0301–P0308). Swap with another coil — if the misfire moves, replace. ($50–$400 per coil, common)
3. Vacuum leak — Hissing sound, lean codes (P0171/P0174), high idle. Often a torn intake boot or cracked PCV hose. ($30–$400, common)
4. Dirty throttle body or idle air control valve — Carbon buildup on the throttle plate. Clean with throttle body cleaner — $10 fix on older cars. ($10–$150, common)
5. Dirty or failed fuel injector — Causes consistent misfire on one cylinder that doesn't move when you swap coils. ($100–$500 per injector, less common)