P2684: Supply C open-load is tested through HSO7 at powerdown

For P2684, this transmission controller uses high-side output seven, HSO7, for the generic actuator-supply-C open-load diagnosis. During shutdown it disables the high-side output and enables a corresponding low-side output. The connected load should then provide a path that lowers the observed high-side voltage.

What a voltage that stays high means

A level above 40% of battery voltage for at least 60 ms meets the listed failure condition. The result is saved in nonvolatile memory and turned into a DTC at the next startup. Thus a startup code can describe the previous powerdown rather than a fault first measured at that instant.

The battery must remain sufficiently stable during the test; the table limits its change to less than 1.5 V. P0882 or P0883 prevents this evaluation.

A relevant verification event

Evidence that HSO7 operates normally while powered is not enough to verify this shutdown load path. Establish whether the shutdown sequence completed and whether its retained result passed. Use the actual controller’s circuit assignment to identify the output and connected loads. The generic letter C does not identify a universal external actuator, and the summary does not isolate the precise open circuit.

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 937.