HCM System Fault Diagnostic - P0A1D

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

There are 3 parts to the HCM system fault diagnostic. Any of the 3 may cause the diagnostic to be reported as failed. Only if all 3 are passed may the diagnostic be reported as passed. The first reports software execution errors within the HCM software scheduler, system and user stacks, interrupt and input/output subsystems. Once detected, this fault will report an HCM system diagnostic fault for the rest of the current ignition cycle. The second detects a stuck-on situation where all of the inputs indicate that the HCM should be OFF, but the main code is still executing. This could occur, for example, due to an HCM internal hardware fault. Once detected, this fault will report an HCM system diagnostic fault for the rest of the current ignition cycle. The final input is based upon the HCM system voltage. If the supply voltage to the HCM, as measured at the ignition 0 analogue input is out of range a fault flag is raised.

The following DTCs are for internal failures within the HCM:

P1AC8 is the HCM 5-volt reference circuit.
P0A1D is the HCM internal system fault.