An engine misfire happens when one or more cylinders fail to burn their air-fuel mixture properly, so that cylinder stops contributing power for a moment. You feel it as shaking, a rough idle or hesitation, and the engine computer usually logs a code: P0300 for a random or multiple-cylinder misfire, or P0301 to P0308 for a specific cylinder. Most misfires come from the ignition system (spark plugs and coils), the fuel system (injectors), air leaks, or a mechanical problem such as low compression.
Engine misfire symptoms
- Rough or shaking idle, often felt through the seat and steering wheel
- Hesitation, stumbling or loss of power when accelerating
- A check engine light, steady or flashing
- Worse fuel economy and a smell of unburned fuel from the exhaust
- Popping or backfiring sounds
A flashing check engine light means a misfire severe enough to damage the catalytic converter, because unburned fuel overheats it. Ease off, avoid heavy loads and get it diagnosed promptly.
What the misfire codes mean
| Code | Meaning |
|---|---|
| P0300 | Random or multiple-cylinder misfire |
| P0301 to P0308 | Misfire detected in cylinder 1 to 8 (the last digit is the cylinder) |
A single-cylinder code points to something on that cylinder, such as a plug, coil or injector. A P0300 or misfires spread across several cylinders point to something they share: fuel pressure, a vacuum leak, a sensor or a mechanical problem. Cylinder numbering differs by engine; see our firing order guide for common V8, V6 and inline layouts.
Common causes of an engine misfire
Ignition
Worn or fouled spark plugs, a failing ignition coil, damaged plug wires or a cracked coil boot. These are the most common causes and the least expensive to fix.
Fuel
A clogged or leaking injector, low fuel pressure from a weak pump or clogged filter, or a wiring fault to an injector.
Air
Vacuum leaks from cracked hoses or intake gaskets, or a dirty mass airflow sensor, which make the mixture too lean.
Mechanical
Low compression in a cylinder from a burnt or bent valve, worn piston rings, a failed head gasket, or a worn camshaft lobe or collapsed lifter, a known issue on some engines; see 5.7 Hemi tick. Mechanical misfires do not go away with new plugs or coils.
How to diagnose a misfire
- Read the codes with a scan tool and note which cylinders are affected.
- Inspect the plugs on the misfiring cylinder: oil, fuel fouling or a cracked insulator tell a story.
- Swap the coil with a neighboring cylinder. If the misfire moves, the coil is the problem; if not, check the plug and injector.
- Check for vacuum leaks and look at fuel trims and fuel pressure.
- Run a compression or leak-down test if ignition and fuel check out. Low compression points to a mechanical fault.
When a misfire means engine trouble
Coolant loss with white exhaust smoke alongside a misfire can mean a failed head gasket; see signs of a blown head gasket. Very low compression, metal in the oil or loud knocking point to internal damage; how to tell if an engine is blown covers the signs. If a repair would cost more than it is worth, compare it with a replacement engine: browse our engines by engine code and see engine replacement cost.
More guides: bad spark plug symptoms · bad ignition coil symptoms · rough idle · check engine light · spark plug wires · engine sputtering · low compression in one cylinder · check engine light flashing · car shakes when accelerating · how often to change spark plugs.
Trouble codes: P0300 Random/Multiple Misfire · P0301 cylinder 1 misfire · P0302 cylinder 2 misfire · P0303 cylinder 3 misfire · P0304 cylinder 4 misfire · P0305 cylinder 5 misfire · P0306 cylinder 6 misfire · P0307 cylinder 7 misfire · P0308 cylinder 8 misfire · P0351 Ignition Coil A Circuit · P0201 Injector Circuit Cylinder 1 · P0353 Ignition Coil C Circuit · P0354 Ignition Coil D Circuit · P0355 Ignition Coil E Circuit · P0356 Ignition Coil F Circuit · P0350 Ignition Coil Circuit · P0316 Misfire on Startup · all OBD2 codes.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.