P27D0: Valve-four high can also mean a shutdown interaction with another output

P27D0 is assigned to synchronizer actuation valve four in this transmission. During active operation, the source checks output voltage against supply-dependent limits and evaluates estimated current against requested current. Those electrical calculations are broader than simply observing one high voltage.

A separate shutdown route

At powerdown, the controller enables low-side outputs in sequence. A disabled valve-four output reporting a ground-like state when another output is active, followed by the reciprocal response when commands swap, supports an unwanted connection between them. The controller stores this result and reports it at the next startup.

This delayed reporting explains why a code can appear without a continuously abnormal live value immediately afterward.

Separate the evidence paths

Identify whether the event came from an active-driver check or the saved shutdown interaction. The latter needs information about the paired output, not just the valve-four trace. A successful energized actuation cannot by itself verify the shutdown route. The generic solenoid-R designation must be mapped using the applicable controller information. The source defines fault patterns but does not identify a particular bridged wire, failed valve or output driver as the proven repair.

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 928, GM physical PDF page 941.