P0303
PowertrainCylinder 3 Misfire Detected
Cylinder 3 is misfiring, whichever side of the engine that is on yours. The ECU has logged enough missed combustion events on that single cylinder to set a code.
ⓘ Information only. This page provides general educational information about fault code P0303. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0303 mean?
P0303 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 3 Misfire Detected.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Rough or uneven idle, particularly noticeable from a cold start
- • Engine warning light (flashing if severe)
- • Power loss when accelerating
- • Sometimes a 'pop-pop' sound from the exhaust at idle
- • Worse fuel economy until the fault is fixed
Possible causes
Causes commonly associated with P0303, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Spark plug on cylinder 3 fouled, worn, or with the wrong gap
- 2. Coil pack for cylinder 3 has failed or is intermittent
- 3. Injector clogged or sticking shut
- 4. Loss of compression on that cylinder (worn rings, burnt valve, head gasket)
- 5. Loose or damaged HT lead on older engines
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Identify which cylinder is number 3 on your engine, it's not always obvious. Transverse four-cylinders typically number from the timing-belt end; on V6s and V8s the numbering is split across banks. Check a workshop diagram before pulling parts
- 2. Swap the cylinder 3 coil with cylinder 1's. Clear the code, drive briefly, see whether the misfire moves to cylinder 1 or stays on 3
- 3. If the coil swap moves the fault, replace that coil. If not, repeat with the spark plug
- 4. If neither swap moves the fault, you're likely looking at fuelling or compression. Check fuel trim live data and run a compression test
Common questions about P0303
Why is cylinder 3 harder to reach on my car? +
On transverse four-cylinder engines, cylinder 3 (or 4) often sits closest to the bulkhead, with limited working room. Some V6 layouts also place cylinder 3 awkwardly. It's worth checking your specific engine layout before quoting yourself a five-minute job.
Is replacing one plug enough, or should I do all of them? +
If the others are recent, swapping just the suspect one is fine. If the plugs are all the same age and high-mileage, replace the set. They're cheap, and a marginal plug elsewhere often becomes the next misfire shortly after.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →