B1866

Step 1: Perform visual inspection of BCM wiring harness and connectors for obvious damage, chafing, melted insulation, or signs of water intrusion. Pay special attention to areas where harnesses pass through firewalls, near sharp edges, or contact moving components.

Step 2: Disconnect BCM connector and inspect all pins for corrosion, bent pins, or moisture. Use a multimeter to measure voltage on the suspected power supply circuit pin at the harness side with ignition off - should read 0V if fault is cleared by disconnection.

Step 3: With BCM disconnected and ignition off, check resistance between the suspected circuit and known battery positive terminal. Reading less than 10 ohms indicates a short to power in the harness. Isolate sections of harness by disconnecting intermediate connectors to locate fault zone.

Step 4: If no harness fault found, inspect for aftermarket accessories tied into BCM circuits or power distribution system. Check all add-on wiring, especially alarm systems, remote starters, or auxiliary lighting.

Step 5: If all wiring tests normal, suspect internal BCM failure. Install known-good BCM or have original module bench-tested. Clear codes and verify fault does not return with replacement module before final installation.

Step 6: After repair, verify battery voltage is not present on control circuits with ignition off, clear all codes, cycle ignition multiple times, and road test to confirm repair success and no code return.