Engine Misfire — Causes, Diagnosis and Repair
An engine misfire means one or more cylinders are not firing correctly — combustion is incomplete or absent on that cycle. The engine runs unevenly, produces less power and generates raw unburnt fuel in the exhaust. Over time a persistent misfire damages the catalytic converter.
Misfire Fault Codes
When the engine management light illuminates due to a misfire, an OBD2 scan will show codes in the P030x range:
- P0300 — Random/multiple cylinder misfire
- P0301 — Cylinder 1 misfire
- P0302 — Cylinder 2 misfire
- P0303 — Cylinder 3 misfire
- P0304 — Cylinder 4 misfire
The cylinder number tells us exactly where to focus the diagnosis.
Common Causes by Cylinder
If the misfire is in a specific cylinder (P0301–P0306), the fault is almost always one of these:
Spark Plug Failure
Spark plugs have a defined service interval (typically 30,000–60,000 miles for standard copper plugs, 60,000–100,000 miles for iridium or platinum). A fouled, cracked or worn plug fails to ignite the air-fuel mixture reliably. Plug replacement on all cylinders from £45 (most 4-cylinder vehicles).
Coil Pack / Ignition Coil Failure
Modern vehicles use individual coil-on-plug (COP) ignition coils for each cylinder. A failed coil produces a misfire in that cylinder only. We can swap coils between cylinders to confirm — if the misfire code moves to the new cylinder, the coil is confirmed faulty. Coil replacement from £65 per coil.
Fuel Injector Fault
A clogged or leaking injector either does not deliver enough fuel (lean misfire) or drips fuel when closed (rich misfire). Injector cleaning or replacement from £85 per injector.
Common Causes of Random Misfire (P0300)
Vacuum Leak
An air leak in the intake system causes a lean mixture across all cylinders, leading to a random misfire. A hissing sound from the engine bay and a rough idle that improves when revved are characteristic signs. Vacuum hose replacement from £45.
Low Fuel Pressure
A failing fuel pump or blocked fuel filter causes low fuel pressure, resulting in a lean condition and misfire particularly under load. Fuel pressure testing from £55.
EGR Valve Fault
A stuck-open EGR valve introduces too much exhaust gas into the intake, diluting the mixture and causing misfire at idle. From £95 to clean or replace.
How We Diagnose a Misfire
- OBD2 scan — identify the cylinder(s) and read all stored codes
- Live data — misfire counter by cylinder, fuel trims, O2 sensor behaviour
- Physical inspection — plug condition, HT leads (where fitted), coil resistance
- Coil swap test where applicable
- Compression test if the above does not identify the fault
Book Misfire Diagnosis in Reading
Call 07305 523333. We carry coil packs and spark plugs for most common vehicles on the van. Many misfires are diagnosed and repaired in a single visit. We cover Reading and all Berkshire towns.
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Frequently asked questions
What does an engine misfire feel like? +
An engine misfire feels like a rough, shuddering or juddering engine — as if the car is struggling or stumbling. It may be constant or intermittent. You might notice it more at idle, on acceleration or at specific speeds. In severe cases the engine management light flashes, indicating a catalytic converter-damaging misfire.
Is a misfiring engine safe to drive? +
A mild misfire may be driven carefully for a short distance, but continuing to drive with a severe or constant misfire risks damage to the catalytic converter (raw unburnt fuel damages the substrate). A flashing engine management light means a catalytic converter-damaging misfire is occurring — stop driving and call a mechanic.
What are the most common causes of an engine misfire? +
The most common causes are worn spark plugs, a faulty coil pack or ignition coil, a leaking or clogged fuel injector, a failed crankshaft or camshaft position sensor, a vacuum leak, or low compression in one cylinder. An OBD2 scan identifies which cylinder is misfiring.