U2108 – Lost Communications with ABS/TCS Control System U2108 (GM)

By Reinier (Contact Me)
Last Updated 2023-10-02
Automobile Repair Shop Owner

Table of Contents

1. What Does Code U2108 Mean?
2. Where is the U2108 sensor located?
3. What are the common causes of code U2108?
4. What are the symptoms of code U2108?
5. Get Help with U2108

What Does Code U2108 Mean?

SPECIAL NOTES: Some online sources, including one known official GM source list code U2108 with the following definition “U2108 – “CAN Bus Error ABS (4L30-E Transmission) Conditions”, but it is not known how, when, why, or where this definition originated. Nonetheless, it is important to note that the 4L30-E transmission variant referenced in this definition is/was hugely problematic and unreliable, so it is probable that some failure modes on this transmission sometimes triggered or contributed to the setting of code U2108 on some GM products that date from the early 2000s. 

Based on the above, we strongly recommend that you verify the transmission variant on affected vehicles from this era if code U2108 is present on older GM products before attempting a diagnosis of code U2108. Thus, if an affected vehicle is fitted with the 4L30-E transmission, the wisest course of action would be to seek professional assistance from a specialist transmission repairer to eliminate or confirm the transmission as the most likely cause of code U2108. END OF SPECIAL NOTES.

OBD II fault code U2108 is a manufacturer-specific trouble code that is (most commonly) defined by carmaker General Motors as U2108 – “Lost Communications with ABS/TCS Control System” and is set when the BCM (Body Control Module) detects that it has lost communication with the Electronic Brake Control Module (EBCM).

All modern vehicles, including General Motors products, use up to 50 (or sometimes more) control modules that monitor, control, and manage every possible aspect of the vehicle’s operation, and while all the control modules on any given vehicle are connected with each other via serial communications networks, some networks, such as CAN bus networks, are “stand-alone” networks that monitor or control critical functions.

We need not delve into the fine details of how the various communications networks on modern vehicles interact with each other here, but suffice it to say that the information some networks exchange is more important to the vehicle’s safe operation than other types of information. For instance, the transmission of safety-critical information about the operation of the vehicle’s ABS system and other safety systems like Traction and Stability Control, which use the ABS system to work, will always take precedence over non-safety critical information.

In terms of operating principles, all control modules that share a communications network have unique identifiers that all other modules on the network recognize. Moreover, all the control modules on a shared network are also a) programmed to transmit a specified type of information periodically to all the other modules and b) to acknowledge receipt of such specified information transmitted by all the other modules on the network.

While this arrangement might sound cumbersome, it is nevertheless an effective method of ensuring that all the control modules on a shared communications network remain in contact with all the other modules on the network. For instance, if the traction control module stops working for any reason, all the other modules on the network will report the malfunctioning module to the BCM and each other when its periodic transmissions fail to arrive at modules that are directly associated with the traction control module.

This reporting function is programmed into each control module on a shared network as a safety precaution since the failure of one control module on a shared network may affect the operation of some or all control modules on that network. For example, if the traction control module or the wiring between the traction control module and the BCM fails, the ABS module (or BCM) will typically disable closely related systems like stability control, adaptive cruise control, lane keep assist, and electronic brake distribution systems. However, as a general rule, whether these systems are disabled largely depends on the nature of the failure. In some cases, the BCM might even disable the ABS control module altogether, which will automatically disable all the safety systems that depend on the ABS system to work, although normal braking will remain available.

From the above, it should be clear that effective and rational communication between all safety-critical control modules is essential for the vehicle’s safe operation. Thus, if any failure, defect, or malfunction occurs that prevents or impedes effective communication between the BCM and the ABS control module for longer than one second, the BCM will recognize that it cannot control multiple safety-critical functions effectively, and it will set code U2108 and illuminate a warning light as a result.

Where is the U2108 sensor located?

This schematic diagram shows the location (circled) of the ABS control module on a Chevrolet Captiva model.

Note that although ABS control modules can be replaced, we do not recommend that non-professional mechanics attempt this procedure, because purging air from the brake system, as well as programming an ABS module may require the use of GM-specific equipment and software, neither of which is freely available in the aftermarket.

Therefore, we strongly recommend that when safety-critical trouble codes such as U2108 are present on GM vehicles, you seek professional assistance from suitably qualified and experienced persons to diagnose and resolve such trouble codes. Failure to seek professional assistance could result in sudden and possibly catastrophic brake system failure(s).

What are the common causes of code U2108?

Some common causes of code U2108 could include one or more of the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring or electrical connectors in one or more serial communications systems or networks
• Blown fuses or fusible links
• Defective or malfunctioning ABS control module
• Corrupted or damaged ABS module software
• Failed or failing BCM

What are the symptoms of code U2108?

Some common symptoms of code U2108 could include one or more of the following-

• Stored trouble code and an illuminated warning light
• Depending on the nature of the problem, one or more additional codes could be present along with U2108
• ABS braking may not be available
• Safety systems like traction control, stability control, adaptive cruise control, and others may be deactivated or not available
• The brake pedal may have a “spongy” feel
• Braking performance may be diminished

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