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Engine-Codes – GMC – U18AC GMC

U18AC GMC Code - Transmission Range Control Module Lost Communication With Chassis Control Module On Chassis Expansion CAN Bus

Quick Answer

The U18AC GMC code indicates that there is a communication problem between the Transmission Range Control Module (TRCM) and the Chassis Control Module (CCM) on the Chassis Expansion CAN Bus. Common causes include Faulty wiring or connectors, Faulty Transmission Range Control Module (TRCM), Faulty Chassis Control Module (CCM). 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
U18AC GMC indicates Transmission Range Control Module Lost Communication With Chassis Control Module On Chassis Expansion CAN Bus.
Common causes
Common causes include Faulty wiring or connectors, Faulty Transmission Range Control Module (TRCM), Faulty Chassis Control Module (CCM).
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 13, 2024
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Code U18AC GMC Description

The U18AC GMC code indicates that there is a communication problem between the Transmission Range Control Module (TRCM) and the Chassis Control Module (CCM) on the Chassis Expansion CAN Bus. The Transmission Range Control Module is responsible for controlling the transmission's shifting behavior based on various inputs, while the Chassis Control Module manages various chassis-related functions such as stability control and traction control. When these modules are unable to communicate properly, it can lead to a range of issues affecting the vehicle's performance and safety.

OBDII Code U18AC GMC - Transmission Range Control Module Lost Communication With Chassis Control Module On Chassis Expansion CAN Bus
U18AC GMC Code - Transmission Range Control Module Lost Communication With Chassis Control Module On Chassis Expansion CAN Bus

Common Causes of U18AC GMC

  1. Faulty wiring or connectors
  2. Faulty Transmission Range Control Module (TRCM)
  3. Faulty Chassis Control Module (CCM)
  4. Software glitches in either module
  5. Issues with the Chassis Expansion CAN Bus

Symptoms of U18AC GMC

  1. Erratic shifting behavior
  2. Transmission slippage
  3. Harsh shifting
  4. Transmission getting stuck in a gear
  5. Issues with stability control or traction control

How to Fix U18AC GMC

  1. Perform a thorough diagnostic scan to identify the exact cause of the communication issue.
  2. Inspect wiring and connectors for any damage or corrosion, repairing or replacing as necessary.
  3. Test the Transmission Range Control Module (TRCM) and Chassis Control Module (CCM) for proper functionality, replacing any faulty modules.
  4. Update software in the modules if needed to address any glitches.
  5. Clear the code and perform a test drive to ensure the issue has been resolved.

Cost to Fix U18AC GMC

The typical repair costs for addressing the U18AC GMC code can vary depending on the specific cause of the communication issue. Generally, the cost of parts such as modules or wiring components can range from $200 to $800, while labor costs can range from $150 to $300. Overall, the total repair cost could fall between $350 and $1100. It is advisable to consult with a trusted auto repair shop to get an accurate estimate based on your vehicle's make and model, as well as the local labor rates.

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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 U18AC GMC Code

1. What are the common symptoms of OBDII code U18AC GMC?

The common symptoms of code U18AC include transmission shifting issues, loss of communication between the transmission range control module and chassis control module, and the illumination of the check engine light.

2. What causes the OBDII code U18AC GMC to occur?

The code U18AC typically occurs due to a communication error between the transmission range control module and the chassis control module on the chassis expansion CAN bus. This can result from wiring issues, a faulty control module, or a malfunction in the CAN bus network.

3. How serious is the OBDII code U18AC GMC?

Code U18AC can affect the proper operation of the transmission system, leading to potential drivability issues. It is important to address this code promptly to ensure the vehicle functions correctly and to prevent further damage.

4. Can I continue driving with the OBDII code U18AC GMC active?

It is not recommended to continue driving with the U18AC code active, as it can impact the transmission's performance and overall drivability of the vehicle. It is best to address the issue as soon as possible.

5. How can I diagnose and repair the OBDII code U18AC GMC?

Diagnosing the U18AC code involves checking the wiring connections between the transmission range control module and chassis control module, testing the control modules for functionality, and inspecting the CAN bus network for faults. Repair may involve repairing or replacing damaged wiring, replacing faulty control modules, or addressing issues with the CAN bus network.

6. Are there any common DIY repair tips for fixing the OBDII code U18AC GMC?

While diagnosing and repairing code U18AC may require specialized tools and knowledge, some DIY enthusiasts may attempt to check the wiring connections, inspect for visible damage, and reset the codes using an OBDII scanner. However, for a thorough repair, it is recommended to seek professional help.

7. How much does it cost to fix the OBDII code U18AC GMC?

The cost of fixing the U18AC code can vary depending on the root cause of the issue. Repair costs may include labor charges for diagnosis and repair, the cost of replacement parts such as control modules or wiring harnesses, and any additional fees for reprogramming modules.

8. Can a low battery voltage trigger the OBDII code U18AC GMC?

Yes, a low battery voltage can potentially cause communication errors between control modules, including the transmission range control module and chassis control module. It is essential to ensure the vehicle's electrical system is in good condition to prevent such issues.

9. Will resetting the OBDII code U18AC GMC make it go away permanently?

Resetting the U18AC code may temporarily clear the fault code, but if the underlying issue causing the code is not addressed, it is likely to reappear. It is crucial to diagnose and repair the root cause of the problem to prevent the code from returning.

10. How long does it take to fix the OBDII code U18AC GMC?

The time required to fix the U18AC code can vary based on the complexity of the diagnosis and repair needed. Simple issues like loose connections may be resolved quickly, while more intricate problems such as faulty control modules or wiring harnesses may take longer to pinpoint and repair.

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