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P0308 2018 GMC Sierra Code - Cylinder 8 Misfire Detected

Quick Answer

The OBD-II code P0308 for a 2018 GMC Sierra indicates that a misfire has been detected in cylinder 8 of the engine. Common causes include Faulty spark plug in cylinder 8, Malfunctioning ignition coil for cylinder 8, Clogged or dirty 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 2018 GMC Sierra indicates Cylinder 8 Misfire Detected.
Common causes
Common causes include Faulty spark plug in cylinder 8, Malfunctioning ignition coil for cylinder 8, Clogged or dirty 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 Sep 17, 2024
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Code P0308 2018 GMC Sierra Description

The OBD-II code P0308 for a 2018 GMC Sierra indicates that a misfire has been detected in cylinder 8 of the engine. A misfire occurs when the fuel-air mixture in the cylinder fails to ignite at the proper time, causing the engine to run rough, lose power, and potentially emit excessive emissions. In this case, cylinder 8 is specifically identified as the source of the misfire.

OBDII Code P0308 2018 GMC Sierra - Cylinder 8 Misfire Detected
P0308 2018 GMC Sierra Code - Cylinder 8 Misfire Detected

Common Causes of P0308 2018 GMC Sierra

  1. Faulty spark plug in cylinder 8.
  2. Malfunctioning ignition coil for cylinder 8.
  3. Clogged or dirty fuel injector in cylinder 8.
  4. Vacuum leak affecting cylinder 8.
  5. Low compression in cylinder 8 due to worn piston rings or valves.

Symptoms of P0308 2018 GMC Sierra

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

How to Fix P0308 2018 GMC Sierra

  1. Begin by diagnosing the specific cause of the misfire in cylinder 8 using a scan tool to identify the underlying issue.
  2. Inspect and replace the spark plug in cylinder 8 if it is found to be faulty or worn out.
  3. Test the ignition coil for cylinder 8 and replace it if it is malfunctioning.
  4. Clean or replace the fuel injector for cylinder 8 to ensure proper fuel delivery.
  5. Check for vacuum leaks in the intake manifold or vacuum lines and repair as needed.

Cost to Fix P0308 2018 GMC Sierra

The typical repair costs for addressing a misfire in cylinder 8 of a 2018 GMC Sierra can vary depending on the specific cause of the issue. In general, the cost of replacing a spark plug or ignition coil can range from $50 to $200 per part, while cleaning or replacing a fuel injector may cost between $100 and $500. Labor costs at auto repair shops can range from $80 to $150 per hour, and the total repair bill could amount to several hundred dollars or more, depending on the extent of the repairs needed. It's recommended to obtain a detailed estimate from a qualified mechanic to determine the exact cost for resolving the misfire in cylinder 8.

What does this mean?

Tech Notes for P0308 2018 GMC Sierra

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 Chevrolet Models:

2000-2020 Chevrolet 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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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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