ENGINE SYSTEM / MISFIRE

Engine misfire codes

A misfire code means the engine computer saw one or more cylinders fail to burn their fuel properly. P0300 means random or multiple cylinders. P0301 through P0308 name the cylinder.

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Start here

If the check engine light is flashing, slow down and follow your owner’s manual. A flashing light while driving usually means a misfire bad enough to damage the catalytic converter.

What to do when the light flashes

Manufacturers agree that a flashing light needs action now, but their instructions differ. Nissan’s 2018 Sentra manual says to stay under 45 mph, avoid hard acceleration and steep hills, and reduce any load you’re carrying or towing. Honda’s 2022 Insight guide says to stop somewhere safe with nothing flammable nearby and wait at least 10 minutes. Ford warns that misfires can push exhaust temperatures high enough to damage parts and create a fire risk.

Follow the instructions in your own manual. If the car shakes badly or loses power, pull over and arrange help.

Read the code number

P0300 means the misfires were random or spread across more than one cylinder. P0301 means cylinder 1, P0302 cylinder 2, and so on. GM’s misfire bulletin, for example, covers P0300 through P0308.

Cylinder numbering depends on the engine, so confirm where cylinder 1 is in the service information before anyone touches a coil or plug.

Three kinds of causes

A cylinder needs spark, the right amount of fuel and good compression. Ignition problems include worn plugs and failing coils. Fuel problems include a clogged or leaking injector, or a mixture that is too lean. Mechanical problems include a valve that doesn’t seal or low compression.

One cylinder with repeated misses points toward something at that cylinder. Misses spread across several cylinders point toward something they share, such as fuel pressure, an air leak or carbon on the valves.

What to save from the scan

Copy every misfire code and whether it is pending or stored. Save the freeze frame, which shows engine speed, load and coolant temperature when the code set. A cold-start miss and a miss under load on the highway lead to different tests. Many scan tools can show misfire counts by cylinder, and the standardized test results in SAE J1979 can include them too.

Misfire you can feel, but no light

The computer turns the light on only after the misfires meet its detection rules. A mild or occasional miss may show up first as a pending code, or not show at all yet. Ask for a scan that includes pending codes, and describe when the stumble happens.

A vehicle-specific example

GM bulletin 16-NA-383 covers 2000–2025 GM gasoline engines with rough idle, hard starting and misfire codes caused by carbon on the valves or sticking valves. GM says the misfires shouldn’t appear before about 5,000 miles, and that normal diagnosis may not find the cause if the valves aren’t sticking during the test. The bulletin gives its own service steps for covered vehicles.