What does it mean?
A misfire is just what it sounds like; a cylinder that is not firing as it should. The P0303 code is caused by a misfire on the number three cylinder. The Misfire monitor diagnostic is based on variation in crankshaft velocity. The PCM determines this information using the crankshaft position sensor and camshaft position sensor. A misfiring cylinder slows down momentarily, so by monitoring the cam and crank sensor signals, the PCM can calculate when a misfire occurs.
A P0303 code tells you only that a misfire has been detected, not WHY the cylinder is misfiring. That’s why with this code, a thorough diagnosis is mandatory. Misfire Monitor Different manufacturers implement OBD-II requirements slightly differently. The misfire monitor is an OBD-II procedure which identifies a misfire. The misfire monitor is a “continuous” monitor; it is always checking the crankshaft position sensor for a misfire.
It is important to note that the misfire monitor is very reliant on the crankshaft position sensor signal. A faulty crankshaft position sensor will misdiagnose misfires! Ignition Systems Waste spark distributorless ignition systems (DIS) are common on many cars. Identifying and understanding a waste spark DIS can help with troubleshooting a misfire. Your car has a waste spark DIS if there are fewer coils than cylinders.
The system shares a coil with two spark plugs corresponding to companion cylinders – the cylinders which are opposite in the firing sequence. More information on the other systems involved in a misfire can be found in the background section of codes P0300 and P0301.