What Is an Engine Misfire?

An engine misfire occurs when one or more cylinders fail to produce power during a combustion cycle. Each cylinder needs three things to fire: fuel, air, and spark. When any of these is missing or mistimed, that cylinder misfires.

A single misfire event is barely noticeable. But repeated misfires cause rough running, loss of power, poor fuel economy, and can damage your catalytic converter if left unfixed.

Symptoms of an Engine Misfire

Common signs of a misfiring engine:

• Rough or uneven idle — The engine shakes or vibrates noticeably at idle.
• Hesitation or stumble during acceleration — The engine feels like it momentarily loses power.
• Flashing check engine light — A flashing CEL specifically indicates an active misfire.
• Loss of power — Reduced acceleration, especially under load or going uphill.
• Poor fuel economy — Unburned fuel is wasted.
• Exhaust smell — Raw fuel smell from the tailpipe.
• Engine sound changes — Uneven firing order creates a different exhaust note.

OBD-II Misfire Codes

Misfire codes tell you exactly which cylinder is affected:

• P0300 — Random/Multiple Cylinder Misfire Detected
• P0301 — Cylinder 1 Misfire Detected
• P0302 — Cylinder 2 Misfire Detected
• P0303 — Cylinder 3 Misfire Detected
• P0304 — Cylinder 4 Misfire Detected
• P0305 through P0312 — Cylinders 5 through 12

P0300 (random misfire) often indicates a system-wide issue like a vacuum leak or fuel quality problem, while cylinder-specific codes point to issues with that cylinder's ignition coil, spark plug, or injector.

Common Causes

Listed from most to least common:

• Worn or fouled spark plugs — The #1 cause of misfires. Replace every 30,000-100,000 km depending on type.
• Failed ignition coil — Coil-on-plug designs mean one coil per cylinder. When a coil fails, that cylinder misfires.
• Vacuum leak — Cracks in intake manifold gaskets or hoses cause lean misfires across multiple cylinders.
• Clogged fuel injector — Prevents proper fuel delivery to one cylinder.
• Low fuel pressure — Weak fuel pump starves all cylinders under load.
• Low compression — Worn piston rings, burnt valve, or head gasket leak (mechanical issue).
• Timing chain/belt issues — Jumped timing causes widespread misfires.

How to Diagnose a Misfire

Step-by-step diagnostic approach:

1. Scan for codes — Identify which cylinder(s) are misfiring using MyMait.
2. Check misfire counts — Use live data to see which cylinder has the highest misfire count.
3. Swap ignition coils — Move the suspect coil to a different cylinder. If the misfire follows, the coil is bad.
4. Inspect spark plugs — Remove and inspect. Look for fouling, wear, or incorrect gap.
5. Check fuel injectors — Use a stethoscope or noid light to verify each injector is clicking.
6. Test compression — A compression test reveals mechanical issues like worn rings or burnt valves.
7. Check for vacuum leaks — Spray carburettor cleaner around intake fittings. If RPM changes, you found the leak.

Repair Costs

Australian cost estimates:

• Spark plug replacement (set): $80–$250
• Ignition coil replacement: $120–$400 per coil
• Fuel injector replacement: $150–$400 per injector
• Vacuum leak repair: $100–$400
• Fuel pump replacement: $400–$900
• Timing chain replacement: $800–$2500
• Head gasket replacement: $1500–$3500