Cylinder 3 Misfire Detected (P0303)
P0303 indicates that the engine control module (ECM) has detected a misfire in cylinder 3. A misfire occurs when the air-fuel mixture in the cylinder fails to ignite properly, causing a loss of power, increased emissions, and potential damage to the catalytic converter. Common causes include faulty spark plugs, ignition coils, fuel injectors, vacuum leaks, or low compression. Immediate diagnosis is recommended to prevent further engine damage.
01. Most Common Causes
- Faulty spark plug or ignition coil on cylinder 3
- Clogged or leaking fuel injector on cylinder 3
- Vacuum leak near cylinder 3 intake
- Low compression due to worn piston rings, valves, or head gasket
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine misfire, rough idle, or hesitation during acceleration
- Reduced fuel economy and power
- Possible engine vibration or shaking
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data to confirm P0303.
Step 2: Swap spark plug and ignition coil from cylinder 3 to another cylinder; if misfire follows, replace the faulty component.
Step 3: Perform a fuel injector balance test or swap injectors to isolate a faulty injector.
Step 4: Check for vacuum leaks using a smoke machine or propane enrichment test.
Step 5: Perform a compression test on cylinder 3; if low, perform a leak-down test to identify internal engine issues.
04. Commonly Affected Makes
05. 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 if the misfire is mild.
How much does it cost to fix?▼
The cost varies depending on the cause. A spark plug or ignition coil replacement typically costs $50-$150 per part, while fuel injector replacement can be $150-$400. Internal engine repairs are more expensive.