P0303Cylinder 3 Misfire Detected

What it means

The crankshaft speed dips every time cylinder 3 should fire but doesn't. As with any single-cylinder misfire, the fault sits in the short list of things serving that one cylinder: its spark, its fuel, its air seal, or its compression.

Is it safe to drive?

Caution

Severity depends on the light. Steady, with a light stumble: drive gently and get it fixed within days. Flashing: raw fuel is overheating the catalytic converter — back off the throttle and stop driving as soon as it's safe. Left alone, a persistent misfire tends to graduate into a P0420 and a far bigger bill.

Common causes

Most likely first.

• A worn or fouled spark plug in cylinder 3
• That cylinder's ignition coil, or its plug wire on older cars
• A clogged or dead fuel injector on cylinder 3
• A vacuum leak at that cylinder's intake runner or gasket
• Low compression in that cylinder — the least common, tested last

What the live data shows

Cylinder 3's misfire counter rising alone is the confirmation. From there the pattern matters: counts under acceleration implicate the coil or plug arcing under load; counts at warm idle implicate the injector or a runner vacuum leak. The freeze-frame stored with the code captures the conditions from the drive where it set.

A scanner that reads all 139 standard sensors shows you these readings live. ScanMyCar charts them as you drive, keeps the freeze-frame stored with the code, and pulls codes from every module — free, with any cheap Bluetooth adapter.

Typical fixes

Cheapest first, where honesty allows.

• Replace the spark plug in cylinder 3
• Swap that coil with a neighbor; if the misfire moves with it, replace the coil
• Swap-test or clean the injector
• Compression-test if spark and fuel check out