P0304
UniversalCylinder 4 Misfire Detected. This code indicates that the Powertrain Control Module (PCM) has detected a misfire condition in cylinder number 4 of the engine.
Medium SeverityDTC P0304 indicates that the engine's computer has detected that cylinder 4 is not firing correctly, meaning the fuel-air mixture in cylinder 4 is not igniting, or not igniting completely. This typically leads to symptoms like a rough running engine, reduced power, and increased exhaust emissions.
⚠️ Before You Begin (Safety First)
Always ensure the engine is cool before working on it to prevent burns. Disconnect the negative battery terminal before servicing any electrical components to avoid accidental shocks or damage. Wear appropriate eye protection and gloves.
Common Symptoms
- Check Engine Light illuminated or flashing
- Rough engine idle
- Engine hesitation or stumbling during acceleration
- Reduced engine power
- Decreased fuel economy
- Vibrations felt through the steering wheel or seat
- Smell of unburnt fuel from the exhaust
Common Causes
- Fouled, worn, or incorrectly gapped spark plug in cylinder 4
- Faulty ignition coil for cylinder 4
- Clogged or faulty fuel injector for cylinder 4
- Vacuum leak affecting cylinder 4
- Low engine compression in cylinder 4 (e.g., worn piston rings, valve issues)
- Damaged spark plug wire (if applicable)
- Problem with the Powertrain Control Module (PCM)
- Wiring issues to the ignition coil or fuel injector for cylinder 4
Frequently reported on: General/Universal
Diagnostic Steps
- 1. Inspect Spark Plug and Ignition Coil: Carefully remove the ignition coil and spark plug from cylinder 4. Look for signs of wear, fouling, or damage. Replace if necessary. Consider swapping the coil with an adjacent cylinder to see if the misfire moves (confirming a faulty coil).
- 2. Check Fuel Injector: While more difficult, you can listen to the fuel injector for cylinder 4 with a mechanic's stethoscope to ensure it's clicking, indicating it's firing. A more advanced test involves checking fuel injector resistance or pulse with a multimeter/oscilloscope.
- 3. Check for Vacuum Leaks: Visually inspect vacuum lines around cylinder 4 for cracks or disconnections. You can also use a smoke machine or unlit propane torch to detect leaks (engine RPM will momentarily increase when propane/smoke is drawn in).
- 4. Perform a Compression Test: If the above steps don't resolve the issue, perform a compression test on cylinder 4 to check for internal engine problems. Compare results with other cylinders and manufacturer specifications.
- 5. Clear Code and Test Drive: After any repair, clear the P0304 code using an OBD-II scanner and test drive the vehicle to ensure the misfire is resolved and the code does not return.
👨🔧 When to Call a Professional
If basic checks and part replacements (like spark plugs or ignition coils) do not resolve the P0304 misfire, or if symptoms are severe (e.g., strong smell of raw fuel, catalytic converter glowing red), it's best to consult a professional mechanic. Complex diagnostics, fuel system issues, or internal engine problems often require specialized tools and expertise.
Commonly Asked Questions
What does a P0304 code mean for my car?
A P0304 code means your car's computer has detected that cylinder number 4 is misfiring, meaning the fuel-air mixture isn't igniting properly.
Can I drive with a P0304 code?
While you might be able to drive short distances, it's not recommended. A persistent misfire can damage your catalytic converter and other engine components, and will reduce power and fuel economy.
What's the most common cause of a P0304 misfire?
The most common causes are a faulty spark plug or ignition coil in cylinder 4. Less common but still possible are a bad fuel injector, vacuum leaks, or internal engine issues.
How can I identify cylinder 4 in my engine?
Cylinder numbering varies by manufacturer. Typically, cylinder 1 is at the front of the engine (nearest the timing belt/chain), and they count sequentially. Consult your vehicle's owner's manual or a repair guide for your specific model.
Do I need to replace all spark plugs if only cylinder 4 is misfiring?
It's often a good idea to replace all spark plugs if they are old or due for maintenance, as other cylinders may soon experience similar issues. However, if only one plug is clearly defective, replacing just that one might resolve the immediate problem.