P0304: Cylinder 4 Misfire Detected
TL;DR
P0304 = cylinder 4 misfire. Severity: high. STOP if the light flashes. Top causes: spark plug (35%), ignition coil (30%), injector/compression (35%). Swap the coil/plug to a neighboring cylinder to see if the misfire follows.
Can I keep driving with P0304?
IF the light is steady → drive gently and fix soon. IF it is FLASHING → stop as soon as safe; an active misfire can quickly destroy the catalytic converter.
Symptoms
- Check engine light (steady or flashing)
- Shaking / rough idle
- Power loss and hesitation
- Worse fuel economy
- Possible unburned-fuel smell
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Worn or fouled spark plug (cylinder 4) | 35% | |
| Clogged injector or low compression (cylinder 4) | 35% | |
| Failing ignition coil (cylinder 4) | 30% | Swap-test to confirm |
What does P0304 mean?
Technical explanation
P0304 sets when cylinder 4 exceeds the ECM’s misfire threshold (measured via crankshaft-speed variation). Diagnosis focuses on cylinder 4’s ignition, fuel and mechanical condition. Swapping the coil or plug to an adjacent cylinder and watching whether the code follows isolates the fault quickly; persistent misfire after ignition parts points to the injector or compression.
In simple terms
P0304 means power chamber number 4 keeps misfiring, making the engine shake and lose power. It’s usually that cylinder’s plug or coil — confirm by swapping with the next cylinder over and seeing if the misfire moves.
How to diagnose P0304 (step by step)
- Confirm P0304 and companions. Cylinder 4; note lean/rich or P0300 codes.
- Swap the coil to an adjacent cylinder. If the misfire follows the coil, replace it.
- Inspect the spark plug. Check cylinder 4’s plug for wear/fouling.
- Test the injector. Confirm cylinder 4’s injector fires and is clean.
- Check compression. Rule out a mechanical fault on cylinder 4.
Repair options & cost
- Replace the spark plug
- Replace the ignition coil
- Clean or replace the injector
- Repair a mechanical/compression fault
🔧 Doing it yourself? Buy the part: Spark plugs · Ignition coil
| DIY cost | $10–$150 |
|---|---|
| Workshop cost | $80–$500 |
| Repair time | 20 minutes to several hours |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
Scan your car: recommended OBD-II scanners →Vehicle-specific notes
- Confirm which physical cylinder is #4 for your engine.
- Coil-on-plug engines swap-test easily.
- On many 4-cylinder engines, cylinder 4 is at one end of the block — confirm before assuming.
Common mistakes to avoid
- Replacing all parts instead of swap-testing
- Driving with a flashing light
- Overlooking injector/compression
- Wrong plug gap
Frequently asked questions
Can I drive with P0304?
Short-term and gently if the light is steady; not at all if it is flashing, which signals an active misfire that can ruin the converter.
Coil or plug for P0304?
Swap the coil and/or plug from cylinder 4 to an adjacent cylinder. If the misfire moves, the swapped part is at fault.
Where is cylinder 4?
It depends on your engine’s numbering and firing order; look it up for your specific engine.
P0304 summary
| Meaning | Cylinder 4 misfire |
|---|---|
| Severity | High |
| Safe to drive? | Caution — no if flashing |
| Top cause | Spark plug / coil |
| DIY cost | $10–$150 |
| Shop cost | $80–$500 |