P27EC: mode-valve A position does not follow its commanded state

P27EC is a range/performance code for internal electronic transmission range control. The GR10 entry tests whether mode-valve A feedback changes appropriately when solenoid commands and park-servo state require a transition. Despite a stuck-on heading, its continuation includes failures in both transition directions.

Two disagreement patterns

One branch detects high feedback when a low command and confirmed PARK state are present. The other detects low feedback when the command is high and the servo is confirmed out of park. Steady-state and transition fault times differ between branches, and the count must exceed four before reporting. The task is 6.25 ms; the entry is Type A, one trip.

The source requires valid sensor circuits, usable park sensing and available pump/line pressure. Transition delays apply before assessing a requested change. These conditions separate a genuinely failed response from an assessment made too early or without hydraulic support.

Diagnostic interpretation

Review command, feedback, park-servo state and hydraulic availability together. A valid voltage within an electrical band does not prove that the signal transitioned at the right time. Conversely, a separate range-sensor electrical fault can invalidate this performance comparison.

Verification must use the prescribed professional range-control procedure with the vehicle secured. The monitor does not identify a single failed valve or permit bypassing park safeguards to reproduce the event.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 2281. GM diagnostic parameters, PDF page 2282. GM diagnostic parameters, PDF page 2283.