P0303 Code: Cylinder 3 Misfire Detected
The P0303 trouble code means the engine computer detected a misfire in cylinder 3. Cylinder-specific misfires are usually caused by ignition issues (spark plug/coil), fuel delivery problems (injector), intake leaks affecting that cylinder, or mechanical issues like low compression.
What Does P0303 Mean?
A misfire happens when cylinder 3 does not burn the air-fuel mixture consistently. The PCM detects misfires by monitoring slight crankshaft speed changes. The cause is typically something that disrupts combustion: weak spark, improper fuel delivery, an air leak affecting mixture, or a mechanical issue that reduces compression.
Symptoms of P0303
- Rough idle or shaking
- Hesitation or stumbling under load
- Loss of power / poor acceleration
- Hard starting
- Stalling at idle (sometimes)
- Fuel smell from exhaust (sometimes)
- Check Engine Light on (may flash if severe)
Common Causes of P0303
Ignition (Most Common)
- Worn, cracked, or fouled spark plug on cylinder 3
- Failing ignition coil or coil boot
- Damaged plug wire (if equipped)
- Oil or moisture in the spark plug well
Fuel Delivery
- Clogged or failing fuel injector on cylinder 3
- Injector wiring/connector issues
- Low fuel pressure (can contribute to misfires under acceleration)
Air / Vacuum Leaks
- Vacuum leak near cylinder 3 intake runner
- Intake manifold gasket leak
- PCV system leaks
Mechanical
- Low compression on cylinder 3
- Burnt or sticking valve
- Timing issues
- Head gasket issues (less common)
How Serious Is P0303?
Cylinder-specific misfires can start small and become frequent. Continuous misfires can overheat the catalytic converter and lead to expensive emissions repairs. If drivability is poor or the CEL flashes, the issue should be addressed promptly.
How to Diagnose P0303
- Check for other codes (lean codes, fuel trim codes, additional misfire codes).
- Inspect cylinder 3 spark plug for wear, oil fouling, cracks, and gap.
- Inspect ignition coil and boot for cracks, carbon tracking, or oil contamination.
- Check cylinder 3 injector connector and wiring for looseness or corrosion.
- Review fuel trims to see if a lean condition is contributing to misfires.
- Inspect vacuum and intake leaks around the manifold and hoses.
- Compression test if ignition/fuel checks don’t resolve the misfire.
How to Fix P0303 (Most Effective Repairs)
| Repair | Most likely when |
|---|---|
| Replace spark plug (cyl 3) | Plug is worn, fouled, cracked, or gap is incorrect |
| Replace ignition coil (cyl 3) | Coil shows failure signs or spark strength is inconsistent |
| Fix oil/moisture intrusion | Oil in plug well or water intrusion causing intermittent misfire |
| Service/replace fuel injector (cyl 3) | Injector restricted/failing or connector/wiring is faulty |
| Repair vacuum leak / intake gasket | Lean trims or leak symptoms near intake |
| Mechanical repair | Compression is low or valve/timing issues are confirmed |
P0303 Repair Cost Estimates
| Repair | Estimated Cost |
|---|---|
| Spark plug replacement | $30 – $150 |
| Ignition coil replacement | $80 – $350 |
| Fuel injector cleaning | $80 – $200 |
| Fuel injector replacement | $150 – $600 |
| Vacuum leak repair | $50 – $250 |
| Compression/leak-down testing | $120 – $300 |
FAQs
Can I drive with a P0303 code?
Short trips may be possible if symptoms are mild, but driving with a misfire can damage the catalytic converter. If the CEL is flashing or the engine is shaking heavily, driving should be avoided.
What causes cylinder 3 misfire most often?
The most common causes are a worn spark plug, failing ignition coil, or fuel injector issue. Vacuum leaks near the intake can also contribute.
How is P0303 different from P0300?
P0303 points to cylinder 3 specifically. P0300 indicates random or multiple cylinder misfires.