P0303 Code: Meaning, Symptoms, Causes & Fixes
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Causes ranked by likelihood
| Cause | Likelihood | DIY difficulty |
|---|---|---|
| Bad spark plug on cyl 3 — Replace first. | Most likely | 2/5 |
| Failing coil on cyl 3 — Swap-test confirms. | Possible | 2/5 |
| Bad injector on cyl 3 — Test resistance. | Less likely | 2/5 |
Symptoms of P0303
- Rough running
- CEL
- Reduced MPG
How to diagnose P0303
- Confirm the code. Read P0303 with an OBD-II scanner and note the freeze-frame data (speed, temperature, load) recorded when it set.
- Check for companion codes. Look for other stored or pending codes such as P0300, P0304 — they often point to one shared root cause.
- Check: Bad spark plug on cyl 3. Replace first.
- Check: Failing coil on cyl 3. Swap-test confirms.
- Check: Bad injector on cyl 3. Test resistance.
- Clear and verify. After the repair, clear the code and complete a full drive cycle to confirm P0303 does not return.
Can I drive with P0303?
No — sustained misfires damage your catalytic converter.
Related codes
Related symptoms
Frequently asked questions
What does P0303 mean?
P0303 means cylinder 3 misfire detected. P0303 indicates a misfire on cylinder 3. The same diagnostic approach applies: swap-test the spark plug and coil to isolate the failed component.
Can I drive with code P0303?
No — sustained misfires damage your catalytic converter.
What is the most common cause of P0303?
Bad spark plug on cyl 3 — Replace first.
What are the symptoms of P0303?
Rough running, CEL, Reduced MPG.
How much does it cost to fix P0303?
A typical shop repair range is $30–$600, depending on which cause it turns out to be. Cheaper causes such as bad injector on cyl 3 should be ruled out first.