Engine Misfire Codes
Misfire codes are among the most urgent check engine light codes. A misfire means a cylinder isn't firing properly — which can cause rough running, power loss, and rapid damage to the catalytic converter if left unaddressed.
Last reviewed May 2026 · Find This Code Editorial Team
What is an engine misfire?
A misfire occurs when a cylinder fails to produce a proper combustion event during its power stroke. The engine's ECU detects this by monitoring crankshaft rotation speed — a cylinder that doesn't fire causes a tiny but measurable hesitation in crank rotation.
Misfires can be caused by ignition system failures (spark plugs, coils), fuel delivery problems (injectors, fuel pressure), or mechanical issues (low compression, valve problems).
Critical warning: A flashing check engine light indicates an active, severe misfire. Stop driving immediately — raw, unburned fuel is passing through the catalytic converter and can destroy it within minutes.
P0300 vs. cylinder-specific codes
P0300 (random/multiple misfire) means the ECU detected misfires but couldn't attribute them consistently to a single cylinder. This often points to a system-wide issue: bad fuel, a vacuum leak, failing spark plugs across multiple cylinders, or low compression.
P0301–P0306 (cylinder-specific) codes identify which exact cylinder is misfiring. These are easier to diagnose — you can swap the ignition coil and spark plug to an adjacent cylinder to confirm if the misfire follows the part.
Misfire codes covered on Find This Code
Misfire diagnosis tips
- Read the freeze frame data. The RPM, engine load, and temperature when the code set tell you whether it's a cold-start, idle, or wide-open-throttle misfire — pointing to different root causes.
- Inspect existing spark plugs first. Pull and examine each plug — the condition (oil fouled, carbon fouled, cracked electrode, worn tip) tells you what caused the misfire before you buy anything.
- Swap to confirm a coil or plug. Move a suspect ignition coil or spark plug to a different cylinder. If the misfire code changes to a new cylinder number, the component is faulty.
- Do a compression test. A cylinder with low compression won't fire correctly regardless of how good the spark or fuel delivery is. Rule out mechanical causes early.