What causes a misfire code?
Ignition System Causes
The most common cause of misfire codes is worn or fouled spark plugs. Over time, spark plugs erode and the gap widens, requiring more voltage to fire. If the ignition coil can't provide enough voltage, the cylinder misfires.
Faulty ignition coils, spark plug wires, or distributor components (on older cars) can also cause misfires. A coil that's breaking down under load may only misfire when the engine is hot or under acceleration.
- Worn spark plugs (wrong gap, fouled, or damaged)
- Faulty ignition coils or coil packs
- Damaged spark plug wires (cracks, burns, high resistance)
- Distributor cap and rotor issues (older vehicles)
Fuel and Air Delivery Causes
Clogged or failing fuel injectors can cause a lean misfire in one or more cylinders. A weak fuel pump or clogged fuel filter reduces fuel pressure, leading to misfires across multiple cylinders, especially under load.
Vacuum leaks introduce unmetered air into the engine, causing a lean condition that can trigger misfires. Common leak points include intake manifold gaskets, vacuum hoses, and the PCV system.
Mechanical Causes
Low compression due to worn piston rings, bad valve seals, or a blown head gasket can cause persistent misfires. A leak-down test or compression test can confirm this.
Timing issues, such as a jumped timing belt or chain, can cause misfires on multiple cylinders. This is often accompanied by rough running and a loss of power.
- Low compression (rings, valves, head gasket)
- Incorrect valve clearance (on adjustable valvetrains)
- Jumped timing belt or chain
- Worn camshaft lobes (rare but possible)
Common mistakes
- Assuming a misfire code always means the spark plug is bad; it could be fuel, air, or mechanical.
- Replacing all spark plugs and coils when only one cylinder is misfiring; diagnose the specific cylinder first.
- Ignoring a misfire code; prolonged misfiring can damage the catalytic converter and increase emissions.
