P0303 Code: Cylinder 3 Misfire Detected
Quick Answer
Cylinder 3 is misfiring — that cylinder isn't completing combustion the way it should.
At a Glance
What this code means
P0303 means the ECU has detected that cylinder 3 is not producing a proper power stroke consistently. This can be an occasional misfire or a persistent one. The ECU's crankshaft position sensor detects the irregularity in engine speed. Cylinder 3 misfires can come from ignition, fuel, or mechanical failures within that cylinder.
A misfire happens when combustion fails in one cycle. The crankshaft loses speed at that cylinder, which the ECU detects as a misfire event.
Common causes
- Worn or fouled spark plug in cylinder 3
- Failed ignition coil for cylinder 3
- Leaking or stuck fuel injector on cylinder 3
- Low compression from worn piston rings or a damaged valve in cylinder 3
- Intake manifold vacuum leak near cylinder 3
- Head gasket leak at cylinder 3 (less common but serious)
Symptoms you might notice
- Engine vibration or rough idle
- Steady or flashing check engine light
- Hesitation when accelerating
- Slightly lower fuel economy
- Possible rough cold starts
Can you still drive?
A flashing check engine light means stop immediately. A steady light — limit short trips only until repaired. Prolonged misfires cause catalytic converter damage.
How serious is this code?
High if severe or accompanied by a flashing light. Moderate if it's intermittent with a steady light. Low compression on cylinder 3 would indicate a more serious engine issue.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Check for related codes — a companion code may point to the true root cause
- Inspect wiring, connectors, and grounds around the affected sensor or component
- Look for obvious physical damage: cracks, disconnected hoses, or corrosion
- Clear the code, drive the vehicle, and see if it returns — intermittent codes can come from loose connections
- Verify the part is actually faulty by testing it before replacing it
How to troubleshoot it
- Inspect and test the cylinder 3 spark plug — replace if worn, oily, or fouled
- Perform the ignition coil swap test — swap the cylinder 3 coil with another and check if the misfire code follows
- Test cylinder 3 compression and compare to the other cylinders
- Check the cylinder 3 injector for proper pulse and fuel delivery
- Inspect the intake manifold gasket near cylinder 3 for vacuum leaks
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner
- Compression tester
- Spark plug socket set
- Coolant pressure tester (if head gasket suspected)
Estimated repair cost range
Spark plug: $5–$25 per plug. Ignition coil: $50–$150. Fuel injector: $150–$350. Head gasket repair (if needed): $1,000–$2,500.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
If cylinder 3 has unusually low compression compared to the others, consider a cooling system pressure test to check for head gasket issues. A sweet smell in the exhaust or white smoke can also point to coolant entering the combustion chamber.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.