P0303 Code: Cylinder 3 Misfire Detected
Last reviewed May 2026 · Reviewed by the Find This Code Editorial Team
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
Common mistakes to avoid
These are the most frequent diagnostic errors when dealing with P0303. Avoiding them can save time and money.
- Replacing only the coil without swapping it first to confirm the misfire follows the component
- Not performing a compression test on cylinder 3 when spark plugs and coils don't resolve the issue
- Ignoring cylinder-specific injector testing when the misfire persists after ignition component replacement
- Overlooking oil fouling on the spark plug as an indicator of a valve seal or ring issue
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.
Learn more
How we write these guides
Find This Code guides are written based on OBD-II specification documentation, established automotive diagnostic practices, and real-world patterns observed across vehicle makes and models. Content is reviewed for accuracy and plain-English clarity before publication. Pages are marked with a “Last reviewed” date and updated when diagnostic guidance or code interpretation changes. All content is educational — it is not a substitute for hands-on diagnosis by a qualified mechanic.
Last reviewed May 2026 · Find This Code Editorial Team · Editorial standards →
Sources & references
Our guides are written based on OBD-II specifications, automotive engineering principles, and established diagnostic best practices. Key reference sources include:
- U.S. EPA — On-Board Diagnostics (OBD)Official EPA documentation on OBD-II standards and emissions monitoring requirements for passenger vehicles.
- ASE — Automotive Service ExcellenceIndustry certification body for automotive technicians; sets best-practice diagnostic and repair standards.
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.