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.
Verifying the Serial Data Circuits using Data Bus Diagnostic Tool
- Using schematics and vehicle build information, identify all low speed GMLAN devices the tested vehicle is equipped with and each device location on the low speed GMLAN serial data bus.
- Ignition ON/Vehicle In Service Mode.
- From Data Circuit input selection, select Terminal 1 to test the low speed GMLAN serial data bus.
- Start the test and verify that all applicable low speed GMLAN devices are displayed under the Control Module column of the Detected State tab.
- If all low speed GMLAN devices are not responding
Refer to step 3 of Circuit/System Verification - Verifying the Serial Data Circuits using Digital Multimeter to test for a short to voltage or short to ground on the low speed GMLAN serial data circuit or an open/high resistance between the X84 Data Link Connector and the first low speed GMLAN device or splice pack.
- If one or more low speed GMLAN devices are not responding
Refer to Circuit/System Testing - Testing the Device Circuits to test the non-communicating low speed GMLAN device closest to the X84 Data Link Connector.
- If all low speed GMLAN devices are responding
- If all low speed GMLAN devices are not responding
- All 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.