Introduction
The P0303 diagnostic trouble code indicates that the powertrain control module (PCM) has detected an engine misfire specifically in Cylinder 3. When this cylinder fails to combust its air-fuel mixture properly, it disrupts smooth crankshaft rotation, triggering a check engine light and potential catalyst damage if left unresolved.
Common Symptoms
- Check Engine Light flashing or glowing steadily.
- Engine shuddering, shaking, or rough idling.
- Noticeable loss of power during acceleration.
- Smell of unburned fuel in the exhaust.
Primary Causes
- Failing Spark Plug: Worn, fouled, or improperly gapped spark plug in Cylinder 3.
- Faulty Ignition Coil: Weak or dead coil pack feeding Cylinder 3.
- Clogged or Defective Fuel Injector: Injector failing to spray fuel properly in Cylinder 3.
- Low Compression: Worn piston rings or leaking valves in Cylinder 3.
Step-by-Step Diagnosis & Fixes
- Swap Ignition Parts: Move the Cylinder 3 spark plug or ignition coil to another cylinder to see if the misfire follows.
- Inspect Spark Plug: Check the removed plug for oil fouling or electrode wear.
- Test Injector Pulse: Use a diagnostic light to verify power and trigger signals at the Cylinder 3 injector.
- Compression Test: Check mechanical engine compression if ignition and fuel components are healthy.