Cylinder 4 Misfire Detected
The engine control module has detected a misfire in cylinder 4.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
A misfire means that cylinder 4 isn't firing correctly, often due to fuel or spark issues. This means your engine isn't running smoothly and might lose power or efficiency.
What the computer is actually seeing
The engine control module monitors the crankshaft position. When it detects an abnormal pattern, like fewer rotations per unit time, it identifies that one of the cylinders isn't firing properly based on predefined thresholds.
What a healthy reading looks like
Generally, you expect consistent RPM and smooth engine idle. If cylinder 4 were firing well, its contribution should match others within 5-10% variance at idle.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 1Inspect wiring and connector for the ignition coil and fuel injector of cylinder 4 for any damage or disconnection. Repair if necessary.
- 2Check the spark plug condition in cylinder 4; replace if it shows wear or is fouled. Expected resistance is 5k-15k ohms for plug resistance.
- 3Test the ignition coil for cylinder 4; measure primary and secondary resistance according to manufacturer specs. Swap with another cylinder's coil to check if misfire follows the coil.
- 4Using a scanner, check live data for cylinder contribution and misfire counts. Clear codes, run the engine, and see if P0304 reappears. If it does, proceed to the next step.
- 5Conduct a fuel injector balance test; swap cylinder 4 injector with another. If misfire changes to the other cylinder, replace the injector. Otherwise, continue down the line.
Common causes
- Faulty spark plug in cylinder 4
- Failed ignition coil for cylinder 4
- Fuel injector malfunction for cylinder 4
- Vacuum leak affecting cylinder 4
- Low compression in cylinder 4
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with a P0304 code?
It's not recommended to drive long-term with a misfire as it can cause catalyst damage and lead to more expensive repairs.
Will this code come back after clearing?
If the underlying issue isn't fixed, yes, it will likely return after several cycles.
Why do I see P0304 with other cylinder misfire codes?
Misfires can be related; multiple codes might indicate a systemic issue like fuel delivery problems or ignition system faults.
Working a real vehicle right now?
Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
Start guided diagnostic →