P0304
PowertrainCylinder 4 Misfire Detected
Cylinder 4 is misfiring. Whichever physical cylinder that is on your engine, the ECU has counted enough missed combustion events there to set a specific code.
ⓘ Information only. This page provides general educational information about fault code P0304. 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 P0304 mean?
P0304 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 4 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:
- • Engine warning light, sometimes flashing during heavier misfires
- • Rough idle that may smooth out at higher revs
- • Hesitation under load, particularly when accelerating uphill
- • Increased fuel consumption
- • Sometimes a slight petrol smell from the tailpipe
Possible causes
Causes commonly associated with P0304, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Spark plug worn, fouled, or incorrectly gapped on cylinder 4
- 2. Coil pack failed or beginning to fail
- 3. Injector blocked or weak
- 4. Damaged HT lead on older engines
- 5. Vacuum leak around cylinder 4's intake port
- 6. Compression loss (rings, valve, head gasket)
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. Read all stored codes first. P0304 alongside P0300 (random misfire) means cylinder 4 is the worst but the others may be borderline, useful context when deciding whether to replace one component or the set
- 2. Inspect cylinder 4's plug, coil, and connector. Visual checks catch a surprising number of faults
- 3. Coil swap test: move cylinder 4's coil to a healthy cylinder, see if the misfire follows
- 4. If the swap doesn't move the fault, run a dry compression test. Cylinder 4 reading 20% lower than the others points at the head
Common questions about P0304
I have P0304 and P0300 stored. Which do I fix first? +
They're related. P0300 means random misfires across multiple cylinders, P0304 means cylinder 4 specifically is the worst offender. Start by fixing what's wrong on cylinder 4, often that clears both codes. If P0300 returns once cylinder 4 is sorted, the other plugs and coils are also borderline.
Can a misfire damage anything if I leave it for a few days? +
A flashing engine light during misfire indicates the catalytic converter is at risk, hot unburned fuel hitting it can destroy the cat in hundreds of miles. A steady light usually allows a few days for diagnosis without major damage, but the underlying cause typically gets worse. Check with a mechanic if symptoms worsen.
Will a fuel additive fix it? +
Almost never. Fuel additives might help marginal injector deposits over time, but they don't fix a worn plug, a failed coil, or a leaking valve. If P0304 is set, the cause is mechanical or electrical.
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 →