|Trouble Code||Fault Location||Probable Cause|
|P0305||Cylinder 5 -misfire detected||Engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM|
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What Does Code P0305 Mean?
A misfire is just what it sounds like; a cylinder that is not firing as it should. The P0305 code is caused by a misfire on the number five 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 P0305 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.
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!
What are the common causes of code P0305 ?
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 P0305 code, the causes could be:
- Cylinder #5 Spark Plug
- Cylinder #5 Coil
- Cylinder #5 Wiring
- Cylinder #5 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 P0305 ?
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.
The following Youtube video describes troubleshooting a GM Vortec engine with a P0305 code:
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