|Trouble Code||Fault Location||Probable Cause|
|P0308||Cylinder 8 -misfire detected||Engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM|
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What Does Code P0308 Mean?
A misfire is just what it sounds like; a cylinder that is not firing as it should. The P0308 code is caused by a misfire on the number eight 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 P0308 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!
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 P0308 ?
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 P0308 code, the causes could be:
- Cylinder #8 Spark Plug
- Cylinder #8 Coil
- Cylinder #8 Wiring
- Cylinder #8 Fuel Injector
TIP: Check for a technical service bulletin. There could be a known repair for a single cylinder code. Sometimes the PCM errors.
What are the symptoms of code P0308 ?
A misfire can cause vibrations at idle. Engine performance including fuel economy will suffer.
How do you troubleshoot code P0308 ?
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 with a good cylinder doesn’t eliminate the original misfire then there could be a problem with compression.
Compression problems can have a number of causes. The most likely are exhaust valves or head gasket leaks. If the engine is overheating or losing coolant there is a strong probability the gasket is damaged.
See articles P0300 & P0301 for in depth troubleshooting of the four misfire causes: fuel, spark, timing, and compression.
Codes Related to P0308
BAT Team Discussions for P0308
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