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Engine-Codes – GMC – C0387 2018 GMC Sierra

C0387 2018 GMC Sierra Code - Unable to Complete Shift

Quick Answer

The diagnostic trouble code C0387 in a 2018 GMC Sierra indicates that the vehicle's computer has detected an issue with the ability to complete a shift. Common causes include Low transmission fluid level, Faulty shift solenoids, Transmission control module (TCM) issues. 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
C0387 2018 GMC Sierra indicates Unable to Complete Shift.
Common causes
Common causes include Low transmission fluid level, Faulty shift solenoids, Transmission control module (TCM) issues.
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 11, 2024
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Code C0387 2018 GMC Sierra Description

The diagnostic trouble code C0387 in a 2018 GMC Sierra indicates that the vehicle's computer has detected an issue with the ability to complete a shift. This typically points to a problem with the transmission system, specifically in the shift control mechanism. When this code is triggered, the vehicle may experience difficulty shifting gears, which can manifest as the transmission getting stuck in a certain gear, delayed shifting, or even complete failure to shift. This can lead to poor acceleration, decreased fuel efficiency, and potential damage to the transmission system if left untreated.

OBDII Code C0387 2018 GMC Sierra - Unable to Complete Shift
C0387 2018 GMC Sierra Code - Unable to Complete Shift

Common Causes of C0387 2018 GMC Sierra

  1. Low transmission fluid level
  2. Faulty shift solenoids
  3. Transmission control module (TCM) issues
  4. Mechanical problems within the transmission
  5. Wiring or electrical issues

Symptoms of C0387 2018 GMC Sierra

  1. Difficulty shifting gears
  2. Transmission slipping
  3. Harsh or delayed shifting
  4. Illumination of the Check Engine Light
  5. Reduced fuel efficiency

How to Fix C0387 2018 GMC Sierra

  1. Begin by checking the transmission fluid level and condition. If low, top up the fluid to the proper level and inspect for any leaks that may be causing the issue.
  2. Perform a diagnostic scan to retrieve the specific trouble codes and pinpoint the exact cause of the shifting issue.
  3. Inspect the shift solenoids, sensors, and wiring harness for any signs of damage or malfunction. Replace any faulty components as necessary.
  4. Test the transmission control module (TCM) to ensure it is functioning correctly. If needed, reprogram or replace the TCM to address the shifting problem.
  5. If the above steps do not resolve the issue, a more in-depth inspection of the transmission system may be necessary, including checking for internal mechanical problems or clutch issues.

Cost to Fix C0387 2018 GMC Sierra

The typical repair costs for addressing a code C0387 in a 2018 GMC Sierra can vary depending on the specific cause of the shifting issue. In general, the cost of parts such as shift solenoids, sensors, or transmission fluid can range from $100 to $500. Labor costs at auto repair shops typically range between $80 and $150 per hour, with the total repair bill potentially exceeding $500 to $1000 or more, depending on the extent of the repairs needed. It is advisable to consult with a professional mechanic for a more accurate cost estimate based on the specific diagnosis and repair requirements for your vehicle.

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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.

Frequently Asked Questions about C0387 2018 GMC Sierra Code

1. What does the OBDII code C0387 mean on a 2018 GMC Sierra?

C0387 indicates an issue where the transfer case control module (TCCM) is unable to complete a requested shift in the transfer case.

2. What are the common symptoms of the C0387 code on a GMC Sierra?

Symptoms may include the "Service 4WD" light illuminating, inability to shift between 2WD and 4WD modes, or unusual noises from the transfer case.

3. What causes the C0387 code to appear on a 2018 GMC Sierra?

Common causes include a faulty transfer case motor, damaged wiring or connectors, a malfunctioning TCCM, or mechanical issues within the transfer case.

4. Can I continue driving my GMC Sierra with the C0387 code?

While the vehicle may still be drivable, it’s recommended to address the issue promptly as it could affect 4WD functionality and lead to further damage.

5. How do I diagnose the C0387 code on my 2018 GMC Sierra?

Diagnosis typically involves scanning for additional codes, inspecting the transfer case motor and wiring, and testing the TCCM for proper operation.

6. How much does it cost to repair the C0387 code on a GMC Sierra?

Repair costs vary but may range from $200 to $1,000 depending on whether the issue is electrical (e.g., wiring or motor) or mechanical (e.g., transfer case damage).

7. What should I check first when troubleshooting the C0387 code?

Start by inspecting the transfer case motor, wiring harness, and connectors for visible damage or corrosion.

8. Can a software update fix the C0387 code on a 2018 GMC Sierra?

In some cases, a TCCM software update may resolve the issue if it’s related to a programming error. Consult your dealership for confirmation.

9. Does the C0387 code always mean the transfer case motor is bad?

No, the transfer case motor is a common culprit, but the issue could also stem from faulty wiring, the TCCM, or internal transfer case problems.

10. How can I prevent the C0387 code from recurring?

Regular maintenance of the transfer case, including fluid changes, and periodic inspections of electrical components can help prevent recurrence.

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