P0303 indicates the PCM has detected misfires on cylinder number three. 

Here are six common problems that may cause OBD trouble code P0303:

  1. Fouled, cracked, worn or otherwise misfiring spark plug
  2. Bad ignition wire - burnt, cracked, carbon tracked wire or plug boot
  3. Defective ignition components - ignition module, ignition coil, distributor cap, etc..
  4. Vacuum leaks
  5. Clogged or otherwise defective fuel injector
  6. Internal mechanical engine fault, ie., low cylinder compression, bad valve, collapsed lifter, etc..

The aforementioned list covers the most common causes of P0303 in the order we feel is most likely to occur based on our past experiences.  Certainly, each situation is different and there are other possible causes for cylinder misfires.  Here are other causes we've seen (in no particular order).

  • Low fuel pressure
  • Leaking fuel pressure regulator
  • Poor fuel quality - water in gas tank
  • Uneven EGR flow caused by carbon build-up on engine with multiple EGR ports
  • loose or corroded fuel injector wiring