Severity: High
Can damage expensive parts or affect drivability. Limit driving and book promptly.
What P0303 means
The engine computer counted misfires on cylinder 3: that cylinder is not burning its fuel completely. A single-cylinder misfire usually points to a part on that cylinder.
Symptoms
- Rough idle or shaking
- Flashing check engine light under load
- Loss of power or hesitation
- Fuel smell from the exhaust
Common causes
Most likely first
- 1Worn spark plug
- 2Failed ignition coil or boot
- 3Clogged or leaking injector
- 4Vacuum leak near that intake port
- 5Low compression (valve, ring or head gasket)
How we diagnose it
- Check misfire counters per cylinder in live data
- Swap the coil to another cylinder and see if the misfire follows
- Inspect plug condition and gap
- Check fuel trims for a lean condition
- Compression or leak-down test if ignition and fuel check out
Every vehicle sets codes under its own conditions. We check the factory diagnostic steps and service bulletins for your VIN, so the repair targets the actual cause.