P0304 Code: Cylinder 4 Misfire Detected
Quick Answer
Cylinder 4 is not completing combustion properly — it's misfiring.
At a Glance
What this code means
P0304 means the ECU detected a misfire specifically in cylinder 4. On a 4-cylinder engine, cylinder 4 is the rearmost cylinder. On V6/V8 engines, its location varies by manufacturer. Like all single-cylinder misfires, it's caused by a failure in the ignition, fuel delivery, or compression within that specific cylinder.
A misfire happens when combustion fails in one cycle. The crankshaft loses speed at that cylinder, which the ECU detects as a misfire event.
Common causes
- Worn or fouled spark plug in cylinder 4
- Failed ignition coil for cylinder 4
- Clogged or leaking fuel injector on cylinder 4
- Low compression in cylinder 4
- Vacuum leak at the intake manifold near cylinder 4
Symptoms you might notice
- Engine vibration felt through the body and steering wheel
- Rough idle, especially on 4-cylinder engines
- Steady or flashing check engine light
- Reduced power and fuel economy
- Possible smell of unburned fuel from exhaust
Can you still drive?
If the check engine light is flashing, stop immediately — catalytic converter damage is occurring. Steady light — limit driving to short trips only.
How serious is this code?
High if the light is flashing. Moderate with a steady light. Single-cylinder misfires are easier to isolate than random misfires.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Do the coil-swap test before buying any part — swapping the coil costs nothing and immediately tells you if the coil is the problem
- Check cylinder 4 compression relative to the other cylinders — a compression drop of more than 20% on one cylinder points to an internal issue, not an ignition problem
How to troubleshoot it
- Inspect the cylinder 4 spark plug — look for fouling, unusual electrode wear, or damage
- Swap the cylinder 4 ignition coil with a known-good cylinder and clear the code — if P0304 becomes P030X on the new cylinder, the coil is confirmed bad
- Perform a compression test on cylinder 4 — significantly lower compression than other cylinders indicates an internal engine issue
- Check the cylinder 4 injector with a noid light or by listening for the injector click during cranking
- Inspect the intake manifold area near cylinder 4 for vacuum leaks
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner
- Compression tester
- Spark plug socket set
- Multimeter
Estimated repair cost range
Spark plug: $5–$25 per plug. Ignition coil: $50–$150. Compression test (shop): $80–$150. Fuel injector: $100–$250.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
Cylinder 4 is the last cylinder fired in most 4-cylinder firing orders. If it shows unusually low compression compared to other cylinders, also consider a cooling system pressure test — coolant intrusion from a head gasket issue can cause localized cylinder misfires.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.