An engine misfire is a combustion failure in one or more cylinders — the air and fuel mixture either fails to ignite, ignites at the wrong time, or burns incompletely. You experience this as rough idle, hesitation under acceleration, vibration through the steering wheel or seat, and in most cases a check engine light. The OBD2 code P0300 indicates a random or multiple cylinder misfire. Codes P0301 through P0308 identify the specific misfiring cylinder. Worn or fouled spark plugs and failed ignition coil packs are the most frequently replaced parts. Clogged or leaking fuel injectors, vacuum leaks, and low compression account for the rest.
A steady check engine light with a misfire code is a non-emergency — diagnose it within a few days. A flashing check engine light means an active misfire is occurring right now and unburned fuel is entering the catalytic converter. At exhaust temperatures, unburned fuel causes rapid overheating of the catalytic converter substrate. Every mile driven with a flashing CEL can add hundreds of dollars in catalytic converter damage on top of the original misfire cause. If the light is flashing, stop driving as soon as safely possible.
Read OBD2 codes first to identify the misfiring cylinder. If a specific cylinder is flagged (P0301 for cylinder 1, P0302 for cylinder 2, etc.), move the ignition coil from that cylinder to an adjacent one and rescan — if the misfire code follows the coil, the coil is faulty. If it stays on the same cylinder, check the spark plug, fuel injector, and compression. Spark plug replacement costs $20 to $120 DIY, $100 to $300 shop. Ignition coil: $30 to $80 DIY, $150 to $350 shop. Fuel injector: $150 to $400 per injector. Compression test to rule out internal engine damage: $80 to $150 at a shop.
The most common causes are worn or fouled spark plugs, failed ignition coils, clogged or leaking fuel injectors, vacuum leaks in the intake manifold, and low compression. An OBD2 scan identifies the misfiring cylinder — P0300 is random, P0301 through P0308 identify specific cylinders.
A steady check engine light with a misfire code is non-emergency — diagnose within a few days. A flashing check engine light means an active misfire sending unburned fuel into the catalytic converter. Every mile driven with a flashing CEL can add hundreds of dollars in catalytic converter damage. If the light is flashing, stop driving immediately.
Spark plug replacement: $20 to $60 DIY, $100 to $250 shop. Ignition coil: $30 to $80 DIY, $150 to $350 shop. Fuel injector: $150 to $400 per injector. If the misfire is caused by low compression, a compression test ($80 to $150 at a shop) determines if internal engine damage exists.
Scan for OBD2 codes — P0301 means cylinder 1, P0302 means cylinder 2, and so on. To isolate whether the ignition coil is bad, swap the coil from the misfiring cylinder to an adjacent one and rescan. If the misfire code follows the coil, the coil is faulty. If it stays on the same cylinder, check the spark plug, fuel injector, and compression.
Reviewed by Vukašin Herbez — Automotive Editor · Last updated 2026-11-05