Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1584 (BODY CONTROL SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC U2106: LOSS OF COMMUNICATON BETWEEN ECM AND FICM OR TCMTEST DESCRIPTION
2003 Chevrolet TrailBlazer 4.2 S, 4WD
Test Description
2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Test Description
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The numbers below refer to the step numbers on the diagnostic table
- 2 .If the fault is a short to ground, a short together, or an open between the ECM and the CAN circuit splices, DTC U2104 will also set. These failures affect the entire system
- 4 .This test is for an open in the SAE J1939 Serial Data Bus+ circuit that will only affect the TCM communications
DTC U2106
| Step | Action | Values | Yes | No |
|---|---|---|---|---|
| Schematic Reference: See appropriate Engine Controls schematic in SYSTEM WIRING DIAGRAMS | ||||
| 1 | Did you perform the Diagnostic System Check-Engine Controls? | - | Go to Step 2 | Go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS |
| 2 | Did DTC U2104 also set? | - | Go to DTC U2104 | Go to Step 3 |
| 3 | 1. Observe the Freeze Frame/Failure Records for this DTC. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Does the DTC fail this ignition? |
- | Go to Step 4 | Go to TESTING FOR INTERMITTENT AND POOR CONNECTIONS |
| 4 | 1. Turn OFF the ignition. 2. Disconnect the ECM and TCM harness connectors. 3. Test the SAE J1939 Serial Data Bus+ circuit for an open between the CAN harness splice and the TCM. Did the resistance measure near the specified value? |
5 | Go to Step 5 | Go to Step 8 |
| 5 | Test the SAE J1939 Serial Data Bus- circuit for an open between the CAN harness splice and the TCM. Did the resistance measure near the specified value? |
5 | Go to Step 6 | Go to Step 9 |
| 6 | 1. Disconnect the FICM. 2. Turn ON the ignition. 3. Probe the SAE J1939 Serial Data Bus+ circuit with a test lamp connected to a good ground. Does the test lamp illuminate? |
- | Go to Step 10 | Go to Step 7 |
| 7 | Test for an intermittent or poor connections at the TCM. Repair as necessary. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS
. Did you find and correct the condition? |
- | Go to Step 13 | Go to Step 11 |
| 8 | Repair the open in the SAE J1939 Serial Data Bus+ circuit between the TCM and the CAN splice. Did you complete the repair? |
- | Go to Step 13 | - |
| 9 | Repair the open in the SAE J1939 Serial Data Bus- between the TCM and the CAN splice circuit. Did you complete the repair? |
- | Go to Step 13 | - |
| 10 | Repair the short to voltage in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? |
- | Go to Step 13 | - |
| 11 | Test the following TCM circuits for an open or a short to ground: The battery positive voltage input circuits. The battery positive voltage output circuits. The ignition voltage input circuits. The ignition voltage output circuits. The switched battery positive voltage circuits. Repair as necessary. Did you find and correct the condition? |
- | Go to Step 13 | Go to Step 12 |
| 12 | Replace the transmission control module. Refer to TRANSMISSION CONTROL MODULE
Allison Did you complete the replacement? |
- | Go to Step 13 | - |
| 13 | 1. Clear the DTCs with a scan tool. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? |
- | Go to Step 2 | Go to Step 14 |
| 14 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? |
- | Go to DTC INDEX | System OK |
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.