Cylinder 4 Misfire Detected
P0304 indicates a misfire detected specifically in cylinder 4: the air-fuel mixture is not igniting reliably in that cylinder. Typical causes are that cylinder's spark plug, ignition coil, or injector, or low compression. This is a high-priority fault, since unburned fuel can overheat and damage the catalytic converter, so locating and repairing the affected cylinder is the priority.
01. Most Common Causes
- The spark plug in cylinder 4 is worn or incorrectly gapped and no longer produces a spark strong enough to ignite that cylinder's mixture.
- The ignition coil (COP) on cylinder 4 is weak or cracked and delivers intermittent high voltage, causing localized misfires.
- The injector for cylinder 4 is dirty or clogged and fails to atomize fuel properly, upsetting that cylinder's air-fuel ratio.
- Low compression in cylinder 4 from burned valves, a head-gasket breach, or worn rings that prevents stable combustion.
- A localized vacuum leak or a fault in the coil or injector harness or connector for cylinder 4 that interrupts the trigger signal.
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine runs rough, especially at idle or under load
- Reduced power and fuel economy
- Possible engine vibration or hesitation
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Swap spark plug and ignition coil from cylinder 4 to another cylinder; if misfire moves, replace the faulty component.
Step 3: Check fuel injector pulse and resistance; clean or replace if faulty.
Step 4: Perform a compression test on cylinder 4; compare to other cylinders.
04. Tools You Will Need
- OBD2 scan tool with live per-cylinder misfire counters
- Compression tester to verify the mechanical sealing of each cylinder
- Spark tester to confirm ignition at each plug wire or coil
- Automotive oscilloscope to analyze coil and injector waveforms
- Digital multimeter to measure the primary and secondary resistance of the coils
1. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances with a misfire, as it can damage the catalytic converter and other engine components. Short trips to a repair shop are acceptable.
How much does it cost to fix?▼
Costs vary depending on the cause. A spark plug replacement may cost $50-$150, while an ignition coil can be $100-$200. Fuel injector or compression repairs can be more expensive.