Cylinder 3 Misfire Detected
P0303 indicates a misfire detected specifically in cylinder 3: 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 3 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 3 is weak or cracked and delivers intermittent high voltage, causing localized misfires.
- The injector for cylinder 3 is dirty or clogged and fails to atomize fuel properly, upsetting that cylinder's air-fuel ratio.
- Low compression in cylinder 3 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 3 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 acceleration
- Increased fuel consumption and possible misfire smell
03. Step-by-Step Diagnosis
Step 1: Scan for codes and note freeze frame data.
Step 2: Inspect spark plug and ignition coil on cylinder 3; swap with another cylinder to see if misfire moves.
Step 3: Check fuel injector operation on cylinder 3 using a noid light or multimeter.
Step 4: Perform a compression test on cylinder 3 and compare to other cylinders.
Step 5: Inspect for vacuum leaks using a smoke machine or propane enrichment.
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 as the misfire can damage the catalytic converter and cause further engine damage. Short trips to a repair shop are acceptable if the engine is not overheating or severely misfiring.
How much does it cost to fix?▼
The cost varies depending on the cause. Replacing a spark plug or ignition coil typically costs $50-$150 per part plus labor. Fuel injector replacement can be $150-$400. Compression repairs can be significantly more expensive.