P0303 — Cylinder 3 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 3.
Read this first
What the P0303 code means
P0303 identifies misfires at cylinder 3. The cylinder is not burning its charge reliably — the cause sits in spark, fuel, air, or compression at that one location.
Single-cylinder misfire codes are among the most diagnosable: isolate and test that cylinder's components, comparing each against a healthy cylinder.
Common symptoms
- Check-engine light on or flashing
- Uneven, rough idle
- Hesitation under acceleration
- Mild power loss
- Rough running that may worsen as the engine warms or under load
Possible causes
- Worn or fouled spark plug at cylinder 3
- Failing ignition coil
- Clogged or leaking fuel injector
- Low compression at that cylinder (valves, rings, or gasket)
- Intake leak at that runner
- Damaged wiring to that coil or injector
Diagnostic considerations
Run the coil-swap test first — it costs nothing on most engines. Then check the plug and injector. A compression test rules out the mechanical causes.
If P0303 appears with other single-cylinder codes on the same coil pack or injector driver group, suspect the shared component rather than three independent failures.
What a technician may inspect
- Plug condition and gap at cylinder 3
- Coil swap test
- Injector operation and balance
- Compression test for cylinder 3
- Connector and wiring integrity at that cylinder
Common mistakes
- Clearing the code and hoping it doesn't return
- Replacing parts at random instead of using the swap test
- Forgetting that a shared coil pack can misfire two cylinders at once
When professional inspection is appropriate
Related codes
Related glossary terms
FAQ
Frequently Asked Questions
What does P0303 mean exactly?
The computer detected repeated misfires at cylinder 3 — its air-fuel charge failed to combust properly often enough to trigger the fault.
Is it safe to drive?
Short, gentle trips to a shop are usually fine with a steady light. A flashing light means stop driving as soon as safely possible — active misfires can overheat the catalytic converter.
Could a dirty injector cause this?
Yes. When spark tests good, a clogged or leaking injector at cylinder 3 is one of the most common remaining causes.
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