P0303 – Cylinder 3 -misfire detected

By Mia (Contact Me)
Last Updated 2017-08-26
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P0303 Cylinder 3 -misfire detected Engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM

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What Does Code P0303 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.

The following Youtube video series describes the GM waste spark distributorless ignition system:

More information on the other systems involved in a misfire can be found in the background section of codes P0300 and P0301.

What are the common causes of code P0303 ?

When you encounter a single cylinder misfire, the causes of systemic misfires such as: fuel pumps, timing problems, EGR valve leaks, or crankshaft position sensors are less likely.  Individual spark plugs, fuel injectors and valves should be carefully examined for the cylinder.  For companion cylinders, the cylinder noted by the PCM may test fine.  Be sure to check its companion in the firing sequence.  For a P0303 code, the causes could be:

  • Cylinder #3 Spark Plug
  • Cylinder #3 Coil
  • Cylinder #3 Wiring
  • Cylinder #3 Fuel Injector

TIP: Check for a technical service bulletin. There could be a known repair for a single cylinder code.  Sometimes the PCM errors.

How do you troubleshoot code P0303 ?

Often, the best way to troubleshoot a single cylinder misfire is to swap ignition components between the misfiring cylinder and a good cylinder.  If the misfire moves with the sparkplug (for example) then the sparkplug would be the cause of the misfire.  If switching components to a good cylinder doesn’t eliminate the original misfire then there could be a problem with compression.  See articles P0300 & P0301 for in depth troubleshooting of the four misfire causes: fuel, spark, timing, and compression.

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