I dont recommend you delay dealing with the P0304 problem as the misfire can eventually damage your engine and the catalytic converter and what could be a simple cheap repair will turn into thousands of dollars worth of repairs.
Diagnosing misfires can be sometimes challenging, but I have a complete step-by-step guide to diagnose misfires for you, as well as the study of 120 vehicles with misfires which will give you ideas about which components are causing it in your vehicle.
P0304 quick overview
| Meaning | P0304: Cylinder #4 Misfire Detected |
| Is it serious? | Yes, this code is serious and should be addressed immediately to avoid costly damage to the engine or catalytic converter. |
| Possible causes | -Worn or faulty spark plug in cylinder #4 -Defective ignition coil for cylinder #4 -Malfunctioning fuel injector for cylinder #4 -Timing issues (e.g., camshaft timing off) -Vacuum leak affecting cylinder #4 -Internal engine issues (e.g., damaged valve or head gasket) -Wiring or electrical connection problems related to cylinder #4 |
| How to diagnose? | Inspect Spark Plug: Check the spark plug in cylinder #4 for signs of wear or fouling. Test Ignition Coil: Swap the ignition coil with another cylinder to see if the misfire moves. Check Fuel Injector: Perform a swap or test of the fuel injector in cylinder #4 to confirm it’s functioning correctly. Inspect Vacuum System: Check for vacuum leaks, particularly around the intake manifold and PCV valve. Perform Compression Test: Conduct a compression test on cylinder #4 to rule out internal mechanical issues. Inspect Timing Components: Examine the timing belt or chain and related components for misalignment or damage. Inspect Wiring: Examine wiring and connections related to cylinder #4 for any faults or loose connections. |
P0304 Meaning
The P0304 code shows that your car’s engine isn’t burning fuel properly in cylinder #4. This can cause the engine to run poorly and might lead to more expensive damage if not fixed soon. Start by checking the spark plug, ignition coil, and fuel injector in that cylinder.
Car Diagnostics Training
- Car Electrical basics
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- OBD scanner buyer’s guide + discounts
Case study: real cases of solving P0304
I checked specifically for code P0304 and picked these 5 example fixes from my full case study of 123 vehicles with misfires.
How to fix P0304 on Buick
Car: 1998 Buick Century Custom 3.1L
Problem: Misfire, P0300, P0304
Diagnosing: The technician inspected the ignition system which was without issues. The diagnosing process was finished by swapping fuel injectors which moved the misfire to another cylinder.
Fix: New fuel injector (cylinder #4)
How to fix P0304 on Suzuki
Car: 2006 Suzuki Forenza Premium 2.0L
Problem: Misfire, P0301, P0304
Diagnosing: Diagnosing was easy as the car has 2 ignition coils and one for cylinder #1 and cylinder #4 which exactly matches the code’s description. The only thing left is to verify it by swapping the ignition coils to see if the misfire moves to the other 2 cylinders.
Fix: New ignition coils fixed the issue.
How to fix P0304 on Nissan
Car: 2005 Nissan Sentra 1.8L
Problem: No Start, Hesitation, Misfire, P0304
Diagnosing: The technician tried replacing the basic stuff like spark plugs and fuel filter which didn’t work, but he was able to discover coolant in cylinder #4.
Fix: Re-machine engine head fixed the issue
How to fix P0304 on Subaru
Car: 2002 Subaru Forester 2.5L
Problem: Misfire, P0301, P0302, P0303, P0304
Diagnosing: The technician checked the timing, MAF, and MAP sensors which all seemed OK. The only clue was negative fuel trim (-24%) indicating the engine is running rich, pointing at a failing ignition system.
Fix: Change ignition coils
How to fix P0304 on Kia
Car: 1999 Kia Sportage 2.0L Misfire
Problem: P0300, P0301, P0303, P0304
Diagnosing: This is the case of a simple issue that can really prolong the diagnosis duration. The technician changed the spark plugs at first which didn’t help and after diagnosing the vehicle further he decided to replace the plugs one more time.
Fix: New set of spark plugs solved it.
The key takeaways from these cases
- Negative fuel trims mean it is trying to correct for a rich condition. Maybe the coolant temperature sensor is sending false info to the PCM. May also be a bad map sensor.
- Remember that new aftermarket (even OEM) parts can come faulty. Don’t forget that in your diagnosing process
P0304 FAQ and conclusion:
Check the Wikipedia article on combustion engines if you are still unsure what exactly the misfire is. Understanding it will help you with your diagnosis.
The key takeaways from these cases
- Don’t change coils/spark plugs unless you confirm they are faulty (do a swap test at least)
- A lot of misfire cases are caused by internal engine damage/timing so don’t rush with replacing ignition/fuel parts without further diagnosis
- Your diagnosing is sometimes right, but only the OEM part will solve the issue
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko