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Error Code P0308 is defined as Cylinder #8 misfire detected. This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II, specifically vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs vary from one make and/or model to another.
For a quick background in the cylinder system, each cylinder is connected to the crankshaft. Sparkplugs are placed on top of each cylinder, which works by using the mixture of flammable fuel/air compressed inside the cylinder and cause a spark, which sets fire to the fuel. The fuel is then released into energy to power the crankshaft. Problems in the cylinders that cause it to not fire properly, will cause the increase or decrease of the ROM of the crankshaft.
Error Code P0308 is set when the PCM (powertrain control module, or also known as ECM or engine control module in some vehicles makes) has detected a misfire problem in the cylinder #8 (cylinder may not be firing properly). Specifically, the PCM has detected an increase or decrease in crankshaft revolutions per minute (RPM) which exceeds 2%.
As with other error codes, this code triggers the Check Engine light as one of its first noticeable signs.
- Hard starting
- Rough idling
- Rough running
- Engine stalls upon acceleration
- Engine stumbles and/or hesitates
- Engine lacks power while driving
- Engine randomly stalls or dies at stop signs
In some rare cases, vehicles may not show any of these symptoms.
Some of the most common possible causes of this error code are:
- Burned valves
- Clogged exhaust gas recirculation (EGR) valves or tubes
- Cracked distributor cap
- Defective wire or spark plug
- Damaged coil (pack)
- Defective O2 (oxygen) sensor
- Faulty fuel injector
- Faulty catalytic converter
- Burned exhaust valve
- Poor compression
- Vacuum leaks
- Head gasket leaks
- Inaccurate ignition timing
- Low fuel pressure or running out of fuel
- Faulty PCM
How to Check
As with other trouble codes, mechanics diagnose vehicles with this error code by recording all trouble codes and their freeze frame data stored in the PCM.
They clear the code and then perform a test drive to see whether the code coms back or note.
Then, they proceed on inspecting the spark plug wire for cylinder 8and look for signs of damages, and excessive wear.
They will also check the coil packs and coil pack wiring and look for signs of breakage, corrosion, wear and tear and other forms of damages.
Then, they will check the distributor cap and rotor button and look for signs of corrosions, excessive wear and other forms of damages (applicable if the vehicle has distributor cap and rotor button system).
They will repair, if not replace, any damaged spark plugs, spark plug wiring, coil packs and/or coil pack wiring, whichever is needing repair or replacement.
Then, they will check the fuel injectors and its wiring to see if the code comes back. They will also take the vehicle for another test drive to see if the code comes back.
If the code returns, they will check the compression system for cylinder 8.
If after all these diagnosis the error code P0308 still comes back, then they will consider checking and replacing the PCM, as it may be the one that is damaged. This is a rare condition, but possible.
How to Fix
Some of the few common repairs are already stated above, however, they may not be enough. For a thorough repair, you may consider these solutions:
- Replace damaged or faulty spark plugs
- Replace damaged spark plug wires
- Replace damaged coil packs and/or wires
- Repair or replace head gasket leaks
- Replace burned valves
- Replace faulty catalytic converter
- Replace faulty fuel injectors
- Replace damaged full throttle position sensor
- Replace damaged O2 (oxygen) sensor
- Replace faulty MAF sensor
- Replace damaged crankshaft sensor
- Replace damaged camshaft sensor
In some cases, the vehicle may need diagnosis and repair for other related trouble codes. Some may need repairing of internal engine components. If the cylinder is damaged, then the engine may be replaced.
In rare conditions, the PCM may need a reprogramming or must be replaced entirely.
Misfiring condition can lead the engine to completely shut down, or not start at all. It can also cause drivability issues, making the vehicle’s operation unreliable, if not hazardous. Thus, Error Code P0308 is considered a serious condition and should be addressed as soon as possible.