Misfires are one of the most common drivability complaints and show up on the OBD-II scanner as codes P0300 through P0308. P0300 indicates a random or multi-cylinder misfire. P0301 through P0308 indicate misfires in specific cylinders (the last digit is the cylinder number). Understanding what causes misfires and how to diagnose them systematically saves you from replacing parts you don’t need.
What Is a Misfire?
A misfire is when a cylinder fails to produce its expected power stroke – either it doesn’t fire at all, fires at the wrong time, or burns fuel incompletely. The ECM detects this by monitoring crankshaft rotation speed – a missing combustion event causes a detectable slowdown. Symptoms include rough idle, shaking at idle, loss of power, poor fuel economy, a flashing check engine light during hard acceleration (which indicates active misfires severe enough to damage the catalytic converter), and sometimes a smell of unburnt fuel from the exhaust.
The Three Causes of Misfires
All misfires come down to one of three things: no spark (ignition system), no fuel (fuel system), or no compression (mechanical). Starting your diagnosis by identifying which of these three is the cause narrows the search dramatically.
Ignition System Causes
Worn or fouled spark plugs are the most common misfire cause and the cheapest fix. See our spark plug guide. Failing ignition coils are the second most common – on coil-on-plug systems, swap the suspect coil with another cylinder’s coil and see if the misfire follows. A faulty ignition coil driver in the ECM is rare but possible. On older distributor-based systems, check the cap, rotor, and leads.
Fuel System Causes
A clogged or stuck-closed fuel injector causes that cylinder to run lean and misfire. Low fuel pressure from a weak pump or clogged filter causes misfires across all cylinders (P0300). A stuck-open injector causes the opposite – rich mixture and misfire. Injectors can be tested by listening with a mechanic’s stethoscope (you should hear a distinct clicking) or by measuring resistance across the terminals (values in your workshop manual).
Mechanical Causes
Low compression in a cylinder causes that cylinder to misfire consistently. Causes include burnt valves, worn rings, a blown head gasket, or timing belt/chain wear causing valve timing to drift. A compression test (see our compression test guide) identifies these quickly. A blown head gasket between two cylinders causes both to misfire and the compression test shows both low.
Multi-Cylinder Misfires (P0300)
When multiple cylinders misfire randomly, the cause is usually something that affects the whole engine: a vacuum leak (unmetered air leaning out the mixture), low fuel pressure, a failing crankshaft or camshaft position sensor causing incorrect timing, or contaminated fuel. See our OBD-II codes guide for more on interpreting related codes.
Diagnostic procedures, injector test specifications, and ignition system layouts are vehicle-specific. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
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