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.
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
Swap the coil or plug with another cylinder
Look for: If the misfire follows the swapped part to its new cylinder, that part is faulty. If cylinder 3 keeps misfiring, look at the injector, wiring, or mechanical condition of that cylinder instead.
Inspect plug color and gap
Look for: Black/sooty = rich, oily = oil fouling, white/blistered = lean or overheating. A gap outside spec (even without visible damage) can cause a weak spark that misfires under load.
Check the injector connector at cylinder 3
Look for: A corroded, loose, or partially unseated connector causes an intermittent or no-pulse injector — look for green corrosion or a pin that doesn't seat fully.
Smoke-test the intake if fuel trims are lean
Look for: Hissing or visible smoke escaping near the cylinder 3 intake runner or gasket confirms unmetered air is leaning out that cylinder specifically.
Compression-test cylinder 3
Look for: A reading well below the other cylinders points to a valve, ring, or head gasket problem — a mechanical cause that ignition or fuel parts won't fix.
This page explains the generic P0303 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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