Conditions for Setting the DTC

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 Chevrolet Tahoe, 2003 Chevrolet Suburban, and 2003 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

When the BCM receives a command to illuminate the low beam headlamps and the headlamp low beam relay control circuit is open or shorted to voltage.
When the BCM does not receive a command to illuminate the low beam headlamps and the headlamp low beam relay control circuit is shorted to ground.
When the low beam relay coil side is open or the battery positive voltage to the low beam relay is open.