Summary
P0303 means the PCM detected repeated misfires in cylinder 3. The most common causes are a worn spark plug (35%), a failing ignition coil (30%), and a faulty fuel injector (15%). Use the coil-swap test to quickly isolate the cause: move cylinder 3's coil to another cylinder, clear codes, and drive. If the misfire follows the coil, replace it ($20–$80). If it stays at cylinder 3, check the plug and injector next.
Severity: Moderate to High — misfires damage catalytic converters
Safe to drive: Not recommended
Repair cost: $20–$500 depending on cause
DIY difficulty: Easy to Moderate
What does P0303 mean?
The PCM detected that cylinder 3 is misfiring beyond the acceptable threshold. The crankshaft position sensor detects a deceleration at the point in the firing order where cylinder 3 should contribute power. Persistent misfires at that position trigger P0303.
On many inline 4-cylinder engines, cylinder 3 is the third from the timing end. On V6 engines, cylinder 3 location varies by manufacturer — it could be on either bank. Always confirm cylinder numbering from your vehicle's service manual before swapping parts.
If P0300 (random misfire) is also present, cylinder 3 may be the worst but not the only misfiring cylinder.