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Engine-Codes – GMC – P0308 GMC

P0308 GMC Code - Cylinder 8 Misfire Detected

Quick Answer

When a GMC vehicle's onboard diagnostic system detects a misfire in cylinder 8, it means that the engine is not functioning properly. Common causes include Worn-out or fouled spark plugs, Faulty ignition coil in cylinder 8, Clogged or malfunctioning fuel injector in cylinder 8. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P0308 GMC indicates Cylinder 8 Misfire Detected.
Common causes
Common causes include Worn-out or fouled spark plugs, Faulty ignition coil in cylinder 8, Clogged or malfunctioning fuel injector in cylinder 8.
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Aug 25, 2024
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Code P0308 GMC Description

When a GMC vehicle's onboard diagnostic system detects a misfire in cylinder 8, it means that the engine is not functioning properly. A misfire occurs when the fuel-air mixture in the cylinder does not ignite at the right time or fails to ignite altogether. This can lead to a loss of power, rough idling, decreased fuel efficiency, and increased emissions. Ignoring this issue can result in damage to the catalytic converter, oxygen sensors, and other engine components. The misfire may be caused by issues such as faulty spark plugs, a malfunctioning ignition coil, fuel injector problems, or engine mechanical issues.

OBDII Code P0308 GMC - Cylinder 8 Misfire Detected
P0308 GMC Code - Cylinder 8 Misfire Detected

Common Causes of P0308 GMC

  1. Worn-out or fouled spark plugs.
  2. Faulty ignition coil in cylinder 8.
  3. Clogged or malfunctioning fuel injector in cylinder 8.
  4. Low compression in cylinder 8.
  5. Vacuum leaks or intake/exhaust valve problems.

Symptoms of P0308 GMC

  1. Rough idling or vibrations while the vehicle is running.
  2. Loss of power or acceleration.
  3. Engine misfires or hesitation during acceleration.
  4. Increased fuel consumption.
  5. Check Engine Light illuminated on the dashboard.

How to Fix P0308 GMC

  1. Diagnose the specific cause of the misfire by performing a compression test, checking spark plugs, ignition coils, and fuel injectors.
  2. Replace spark plugs in cylinder 8 if they are worn out or fouled.
  3. Test and replace the ignition coil in cylinder 8 if it is faulty.
  4. Clean or replace the fuel injector in cylinder 8 if it is clogged or malfunctioning.
  5. Address any engine mechanical issues causing the misfire, such as low compression or valve problems.

Cost to Fix P0308 GMC

The cost of repairing a cylinder 8 misfire in a GMC vehicle can vary depending on the specific cause of the issue and the labor rates in your area. On average, you can expect to pay between $100 and $500 for parts and labor. Keep in mind that diagnostic time and labor rates at auto repair shops can range from $80 to $150 per hour. It's advisable to consult with a local mechanic or dealership for a more precise estimate based on your vehicle's make and model.

What does this mean?

Tech Notes for P0308 GMC

The P0308 code means that cylinder 8 is misfiring or is randomly misfiring. Start by checking for intake leaks and if no leaks are found the next step is to replace the spark plugs on cylinder 8. If the problem persists more tests need to be done to diagnose the problem, see "Possible Causes".

There is a Factory Service Bulletins for All 2000-2020 GMC Models:
2000-2020 GMC Factory Service Bulletin OBDII Code P0308
This Factory Service Bulletin explains how this condition may be caused by significant carbon build-up on the intake and/or exhaust valves due to fuel contamination or incomplete burning of the fuel.

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Information for Specific GMC Models

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

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