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.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
- 2:
A partial malfunction in the high speed GMLAN serial data circuits uses a different procedure from a total malfunction of the high speed GMLAN data circuits. The following modules are connected to the high speed GMLAN serial data circuits:
- Engine control module (ECM)
- Transmission control module (TCM)
- 5: Data link connector terminals 6 and 14 provide the connection to the GMLAN serial data high circuit and the GMLAN serial data low circuit respectively.
- 13: The communication malfunction may have prevented diagnosis of the customer complaint.
| Step | Action | Yes | No |
|---|---|---|---|
| Schematic Reference: Data Link Connector (DLC) Schematics Connector End View Reference: Master Electrical Component List in Wiring Systems |
|||
| 1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
| 2 |
IMPORTANT:
Make sure the CANdi module works fine. When functioning properly, CANdi's LED flashes. In the event of a problem, the LED will be constantly illuminated.
|
Go to Step 3 | Go to Step 5 |
| 3 | Did you record DTC U0101 or U2105? | Go to DTC U0100-U0299 or DTC U2105-U2199 | Go to Step 4 |
| 4 |
IMPORTANT:
Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions.
|
Go to Step 14 | Go to Diagnostic Aids |
| 5 |
|
Go to Step 12 | Go to Step 6 |
| 6 |
|
Go to Step 7 | Go to Step 10 |
| 7 |
|
Go to Step 12 | Go to Step 8 |
| 8 | Inspect for poor connections at the high speed GMLAN serial data circuits of the TCM. Refer to Testing for Intermittent Conditions and Poor Connections
and Connector Repairs
in Wiring Systems. Did you find and correct the condition? |
Go to Step 12 | Go to Step 9 |
| 9 | Replace the TCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? |
Go to Step 12 | - |
| 10 |
|
Go to Step 12 | Go to Step 11 |
| 11 | Replace the ECM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? |
Go to Step 12 | - |
| 12 |
|
Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 13 |
| 13 | Did you record any DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 14 |
| 14 | Use the scan tool in order to clear the history DTCs. Did you complete the action? |
System OK | - |
NO RELATED
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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.