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P0308 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

P0308 is an OBD-II Code that refers to Cylinder 8 Misfire Detected

Meaning

A P0308 code is the result of the ECM (Engine Control Module) receiving a signal that the powertrain control module (PCM) has discovered a problem with the ignition of the fuel supplied to cylinder 8. This usually happens several times before a trouble code is stored and the Check Engine Light illuminates.

Causes

Common causes for this code include:

  • An oxygen sensor malfunction, for example, would cause the engine to misfire.
  • Fuel injector failure
  • Spark plugs, ignition coils, spark plug boots and wires, and distributors can all cause problems.
  • The fuel pressure regulator and/or the EGR valve might be faulty in this situation.
  • A serious vacuum leak has been discovered.
  • A faulty powertrain control module is at the heart of this issue.

Symptoms

The symptoms of a P0308 code are: 

  • The engine may be more difficult to start.
  • The engine may stutter, hesitate, or fail.
  • Decreased fuel economy.

Diagnosis

To diagnose a P0308 DTC code, a technician would:

  1. Verify that cylinder 8 is misfiring.
  2. Check the fuel pressure to make sure that it's within specification.
  3. Confirm that there are no vacuum leaks.
  4. Confirm that all cylinders are firing by using an endoscope or spark plug tester.
  5. Compare the compression readings for different cylinders with known good results to ensure that it's not a faulty cylinder head gasket, which would be common in this situation.
  6. Update the ECM/PCM if necessary with updated software from Workshop Manual
  7. Clear and re-test to see if the code returns after making repairs.  If necessary, test components such as coils and plugs under load conditions.  This means testing them when they're under the influence of other components.

Common mistakes

The following are common mistakes when diagnosing the trouble code P0308:

Assuming that the check engine light is on because of the P0308 code without first checking all systems and components.

Confusing a P0300-P0301-P0302-P0303-P0304 code (misfires on different cylinders) as a P03088 code (unspecific cylinder misfire).

Not performing a spark test under load conditions to determine which components need to be replaced or repaired.

Not following proper service procedures when replacing components, such as using anti-seize compounds when working with oxygen sensors and heat cycles for ceramic insulation found in fuel injectors.

Failure to follow diagnostics according to OBD II diagnostic trouble codes instead of generic specifications outlined in the workshop manual, which might

How serious is this?

A P0308 code is serious enough that it could result in damage to other components if the code is ignored.  If the misfire is sufficient, driveability will be impacted and fuel economy may decrease.  If the misfire is sufficient to cause a lean air/fuel mixture, it could also damage components in the emission system.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Check all wiring and connections that lead to the cylinders if there are symptoms such as an engine stumbling or stalling.
  • It's a good idea to replace the ignition components as part of your normal maintenance program, depending on how long they've been in the vehicle.
  • Ignition wires, spark plug cables, a distributor cap, and a rotor (if applicable) are all good candidates.
  • In rare instances, the catalytic converter has to be replaced. If you detect rotten eggs in the exhaust, your cat converter should be changed.

Related codes

A P0308 is related to and may be accompanied by the following codes:

P0300-P0301-P0302-P0303-P0304.  Misfire codes that indicate a misfire on different cylinders instead of the specific code P0308, which indicates a misfire on cylinder 8.

P0325.  Knocking code that's accompanied by a P03088 or related misfire code on cylinder 8.

P0420-P0430 catalyst system low-efficiency codes, which could be caused by a defective catalytic converter.

How much does it cost to fix the P0308  code?

The repair cost of a P0308 code depends on the severity of the code.  It's typically less expensive to replace parts before they cause additional damage to other components, but it also depends on what needs to be done.  If you have a misfire that results in broken spark plugs or cracks in the distributor cap/rotor, for example, you would just replace those components instead of your entire distributor.

Conclusion

In conclusion, the P0308 code is serious enough that it could potentially damage other components, impact driveability, and decrease fuel economy.  Ignoring this code could lead to additional problems down the road.  This code is best diagnosed and repaired with the help of a professional technician.

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