P0303: Cylinder 3 Misfire Detected
TL;DR
P0303 = cylinder 3 misfire. Severity: high. STOP if the light flashes. Top causes: spark plug (35%), ignition coil (30%), injector/compression (35%). Swap the coil or plug to a neighboring cylinder to confirm whether the misfire follows the part.
Can I keep driving with P0303?
IF the light is steady → drive gently and repair soon. IF the light is FLASHING → stop as soon as safe; the unburned fuel from an active misfire can quickly ruin the catalytic converter.
Symptoms
- Check engine light (steady or flashing)
- Rough idle and shaking
- Power loss and hesitation
- Reduced fuel economy
- Possible fuel smell
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Worn or fouled spark plug (cylinder 3) | 35% | |
| Clogged injector or low compression (cylinder 3) | 35% | |
| Failing ignition coil (cylinder 3) | 30% | Swap-test to confirm |
What does P0303 mean?
Technical explanation
The ECM identifies misfires from crankshaft-speed variations as each cylinder fires; P0303 sets when cylinder 3 exceeds the misfire threshold. Diagnosis targets cylinder 3’s ignition (plug, coil, wiring), fuel (injector) and mechanical condition (compression). Moving the coil or plug to an adjacent cylinder and watching whether the code follows (e.g. to P0304) quickly isolates the fault.
In simple terms
P0303 means power chamber number 3 keeps misfiring, so the engine runs rough and feels weak. Usually it’s that cylinder’s spark plug or coil. You can confirm by swapping parts with a neighboring cylinder and seeing if the problem moves.
How to diagnose P0303 (step by step)
- Confirm P0303 and companion codes. Cylinder 3; note any lean/rich or P0300 codes.
- Swap the coil to a neighbor cylinder. If the misfire follows the coil, replace it.
- Inspect the spark plug. Check cylinder 3’s plug for wear or fouling.
- Test the injector. Confirm cylinder 3’s injector fires and isn’t clogged.
- Check compression. Rule out a mechanical fault on cylinder 3.
Repair options & cost
- Replace the spark plug
- Replace the ignition coil
- Clean or replace the injector
- Repair a mechanical/compression fault
🔧 Doing it yourself? Buy the part: Spark plugs · Ignition coil
| DIY cost | $10–$150 |
|---|---|
| Workshop cost | $80–$500 |
| Repair time | 20 minutes to several hours |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
Scan your car: recommended OBD-II scanners →Vehicle-specific notes
- Confirm which physical cylinder is #3 for your engine — numbering varies.
- Coil-on-plug engines are easiest to swap-test.
- A persistent single-cylinder misfire after new plug/coil suggests an injector or compression issue.
Common mistakes to avoid
- Throwing parts at it instead of swap-testing
- Driving with a flashing light
- Missing a clogged injector or low compression
- Wrong plug gap
Frequently asked questions
Is P0303 serious?
It can be. A steady light is drivable short-term, but a flashing light means an active misfire that can destroy the converter — stop driving and fix it.
How do I find the cause of P0303?
Swap the coil and/or plug from cylinder 3 to an adjacent cylinder. If the misfire moves, the part you moved is bad; if not, suspect the injector or compression.
Which cylinder is number 3?
It depends on your engine’s layout and firing order — check the cylinder numbering for your specific engine.
P0303 summary
| Meaning | Cylinder 3 misfire |
|---|---|
| Severity | High |
| Safe to drive? | Caution — no if flashing |
| Top cause | Spark plug / coil |
| DIY cost | $10–$150 |
| Shop cost | $80–$500 |