Engine Misfire — Symptoms, Codes, and What to Fix First

What's happening

A misfire is a cylinder that doesn't complete a combustion event. The engine fires unevenly, vibrates, and dumps unburned fuel into the exhaust. Modern OBD2 systems detect misfires by tiny changes in crankshaft acceleration and identify the offending cylinder as P0301 (cyl 1), P0302 (cyl 2), and so on. P0300 means the misfire couldn't be pinned to a single cylinder, or multiple are affected.

You might also notice

• Shuddering or jerky idle
• Stumble or hesitation under acceleration
• Loss of power on hills
• Smell of unburned fuel
• Check engine light, often flashing

Likely causes (most common first)

1. Worn spark plugs (most common — replace at the service interval)
2. Failed ignition coil (very common on coil-on-plug engines)
3. Clogged or dead fuel injector
4. Vacuum leak causing lean misfire
5. Low compression (worn rings, burned valve)
6. Bad fuel or water in fuel
7. Stretched timing chain affecting cam timing

What to check first

1. If the cylinder is identified (P030x), swap that coil with a known-good neighbor cylinder
2. If the misfire follows the coil, you've found it — replace that one
3. Pull the spark plug from the misfiring cylinder; look for fouling, wear, or oil
4. Verify fuel pressure on cars where injectors are likely

Common OBD2 codes for this symptom

Don't have the code yet? Look up your code or read it with AXLY.pro.

Can I keep driving?

If the CEL is flashing, no — catalyst damage is happening. If it is steady and the misfire is mild, drivable to the shop or auto-parts store, but fix it this week.

Confirm with the actual code

Symptom-based diagnosis narrows the field — reading the actual stored code finishes the job. AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter and reads every stored DTC.