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 U2104: 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 .A total module failure will set multiple communication codes
- 4 .An open in the SAE J1939 Serial Data Bus+ circuit between the splice and the ECM will affect all communications
- 5 .An open in the SAE J1939 Serial Data Bus- circuit between the splice and the ECM will affect all communications
- 6 .A short across the two CAN circuits anywhere in the system will cause total communications failure.
- 7 .A short to ground anywhere on the SAE J1939 Serial Data Bus+ circuit will affect all CAN communications
- 10 .This test isolates a shorted FICM from a shorted TCM or ECM. By reading the signal from the FICM after the wiring has been tested, without a TCM connected, an FICM signal generating problem can be diagnosed. When the CAN circuits are open between two components, both components generate a 30,000-40,000 Hertz signal. When the system is not open, or more than one module is communcating, the frequency is 2,000-8,000 Hertz
DTC U2104
| 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 any communication DTCs other than DTC U1800, U2104, or U2106 set | - | Go to DTC INDEX | 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 , TCM-if applicable, and FICM harness connectors that contain the CAN circuits. 3. Test the SAE J1939 Serial Data Bus+ circuit for an open between the splice and the ECM. Did the CAN high circuit test open? |
- | Go to Step 16 | Go to Step 5 |
| 5 | Test the SAE J1939 Serial Data Bus- circuit for an open between the splice and the ECM. Did the CAN low circuit test open? |
- | Go to Step 17 | Go to Step 6 |
| 6 | Test for continuity between the SAE J1939 Serial Data Bus+ and the SAE J1939 Serial Data Bus- circuits. Does continuity exist? |
- | Go to Step 15 | Go to Step 7 |
| 7 | Probe the SAE J1939 Serial Data Bus+ circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? |
- | Go to Step 14 | Go to Step 8 |
| 8 | 1. Turn ON the ignition. 2. 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 19 | Go to Step 9 |
| 9 | 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 20 | Go to Step 10 |
| 10 | Probe the SAE J1939 Serial Data Bus- circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? |
- | Go to Step 18 | Go to Step 11 |
| 11 | 1. Reconnect the FICM. 2. Remove the connector pins for the CAN circuits from the ECM harness connectors. 3. Reconnect the ECM connectors. 4. Start the engine. 5. Measure the frequency across the CAN circuits in the harness with a DMM. Does the frequency measure more than the specified value? |
35,000 Hz | Go to Step 12 | Go to Step 22 |
| 12 | Important: If the vehicle is not equipped with a TCM, answer YES to this step. 1. Turn OFF the ignition. 2. Reconnect the TCM-if equipped. 3. Clear any DTCs that are in the memory. 4. Start the engine. 5. Measure the frequency across the CAN circuits in the harness with a DMM. Does the frequency measure more than the specified value? |
2,000 Hz | Go to Step 13 | Go to Step 21 |
| 13 | Test for an intermittent or poor connections at the ECM. Repair as necessary. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS
. Did you complete the repair? |
- | Go to Step 24 | Go to Step 23 |
| 14 | Repair the short to ground in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? |
- | Go to Step 24 | - |
| 15 | Repair the short across the SAE J1939 Serial Data Bus+ and SAE J1939 Serial Data Bus- circuits. Did you complete the repair? |
- | Go to Step 24 | - |
| 16 | Repair the open in the SAE J1939 Serial Data Bus+ circuit between the ECM and the splice. Did you complete the repair? |
- | Go to Step 24 | - |
| 17 | Repair the open in the SAE J1939 Serial Data Bus- circuit between the ECM and the splice. Did you complete the repair? |
- | Go to Step 24 | - |
| 18 | Repair the short to ground in the SAE J1939 Serial Data Bus- circuit. Did you complete the repair? |
- | Go to Step 24 | - |
| 19 | Repair the short to voltage in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? |
- | Go to Step 24 | - |
| 20 | Repair the short to voltage in the SAE J1939 Serial Data Bus- circuit. Did you complete the repair? |
- | Go to Step 24 | - |
| 21 | Replace the TCM. Refer to TRANSMISSION CONTROL MODULE
- Allison. Did you complete the replacement? |
- | Go to Step 24 | - |
| 22 | Replace the FICM. Refer to FUEL INJECTION CONTROL MODULE
Replacement. Did you complete the replacement? |
- | Go to Step 24 | - |
| 23 | Replace the ECM. Refer to ENGINE CONTROL MODULE
Replacement. Did you complete the replacement? |
- | Go to Step 24 | - |
| 24 | 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 25 |
| 25 | 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.