P27C9: Valve-four stuck-off compares failed motion with a working alternate path

P27C9 is shift solenoid four hydraulically stuck off in the cited transmission strategy. The controller’s integrity routine exercises rods three and four with valve four and different selector targets. These are deliberately small movements, not normal complete shifts.

The alternate valve prevents a false shortcut

The valve-four cases use a movement boundary of 100 micrometres: lack of movement beyond that boundary supports ineffective actuation. The continuation requires one of the same rods to move more than 100 micrometres under an alternative valve-three test. This verifies that the position information is capable of changing.

Without that second observation, a flat feedback signal could be confused with a hydraulic valve that stayed off. The code’s conclusion depends on the combined test results.

Check that the diagnostic sequence ran

Recovery logic or a startup gear-state inconsistency can trigger the integrity routine. Verification should preserve completed case identifiers, selector states and rod responses, rather than merely noting that the code did not return on a short drive. The generic solenoid-R description is not a universal hardware map. Use the relevant transmission service procedure for any intrusive tests or component isolation.

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 1017, GM physical PDF page 1018.