P0303
P0303 Cylinder 3 Misfire
P0303 identifies a misfire on cylinder 3. Diagnosis should prove whether the fault follows the coil, plug, injector, or stays with the cylinder.
How urgent is P0303?
Avoid heavy driving if the car is shaking or the check engine light flashes.
Common symptoms
- Rough running
- Hesitation
- Engine vibration
- Poor fuel economy
- Check engine light
Likely causes
- Failed ignition coil
- Worn plug
- Injector issue
- Vacuum leak
- Valve or compression issue
What to check before replacing parts
1
Swap coil or plug with another cylinder
2
Inspect plug color and gap
3
Check injector connector
4
Smoke-test intake if trims are lean
5
Compression test
Use the code with symptoms
A code points at the system, not always the failed part. Add your vehicle, mileage, symptoms, and whether the check engine light is steady or flashing so the AI diagnostic can narrow the next step.