U2106 code — What the code means and how to fix it

Learn what a U2106 code means and how to fix the problem

Because the 2nd position is a “2” it’s a manufacturer-specific code. GM defines a U2106 code as “Lost Comm with Trans Control System.” In other words, the engine control module (ECM) can no longer communicate digitally with the transmission control module. This is most likely the result of a problem with the data line between the two or a failure of a module on the data bus that’s injecting voltage onto the bus.

Possible causes of a U2106 code

• Faulty TCM
• Loss of power to the TCM due to an open in the power wiring or a blown fuse
• Ground connection failure to the TCM
• Short or open in the wiring harness on either the power, ground or data bus between the ECM and TCM
• A failure of any other module on the same data bus that’s causing interference or voltage injection onto the bus.

How the data bus works

Vehicle modules connect to either a high-speed or low-speed Controller Area Network (referred to as a CAN Bus) system. The bus is usually shielded to prevent interference and carmakers often install an end-of-line resistor on each data bus.

Operating information and commands are exchanged among the modules. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits. The modules often send periodic messages that must be confirmed by the receiving module to ensure they are online and receiving and transmitting properly. Each message contains the identification number of the transmitter module.

If a module fails to return a confirmation message, the ECM will set a trouble code and turn on the Check Engine light.

How to diagnose the problem

Since the U2106 code indicates a problem with the TCM, start by checking for good power and ground to the TCM. Get a wiring diagram and use a computer-safe test light or DVOM to check for each.

If the powers and grounds are good, locate the data bus and check for broken or chaffed sections on the wiring harness.

If the wiring harness checks out. Connect your scan tool and try to communicate with the TCM. Then locate each module sharing the data bus and disconnect one module at a time until you see communication return. The module that allows communication is the one causing the data bus to fail. If none of the modules is the problem, then the TCM itself is the likely cause.

Symptoms of a bad TCM and U2106 code

• Check Engine light or the service engine soon warning message showing

• Difficulting shifting, no shifting, hard shifting

• Transmission in limp mode where it won’t shift out of 2nd gear.

• Decrease in power or acceleration, hesitation or stalling

© 2012 Rick Muscoplat