P2685: Supply C low can arise from voltage collapse or excess current

P2685 maps to HSO7 in the cited transmission controller. Its monitor has two failure branches: output voltage below 40% of battery voltage, or current feedback above 5 A. Both contribute to the same circuit-low classification, so concentrating only on a low voltage can miss the reason it set.

A prior test is an enable condition

The shutdown-path test must already be satisfactory. The controller also requires battery voltage above 8.75 V and below 18 V, with no cranking or stop-key state. P0882 and P0883 disable evaluation. Fault confirmation takes at least 60 ms in this continuous monitor.

Those conditions distinguish an eligible HSO7 failure from a broader supply event during a disabled operating state.

Interpret voltage and current together

Retain both measurements and the shutdown-path status when reviewing the fault. A normal voltage captured after the event does not exclude an earlier excess-current branch. Equally, the current threshold is a controller diagnostic criterion, not permission to apply that load during an improvised test. Vehicle-specific service information must identify the connected circuits and safe isolation procedure. The code alone does not select an actuator or controller for replacement.

Document scope and source

2025 GM 25OBDG07A Part 2 documented monitor only; its application-matrix filename join is unresolved. No model, engine or universal calibration applicability is inferred. These figures explain the cited controller logic, not a vehicle-specific repair instruction. Match the vehicle’s service information before applying them.

GM physical PDF page 921.