P27EB: mode-valve A position signal occupies an illegal voltage band

P27EB is generically labeled transmission range control A position sensor circuit/open. The cited internal ETRS monitor is more specific: it detects a raw mode-valve A sensor voltage inside an illegal band, above 1.263 V and below 1.504 V.

Not every invalid input is at a voltage rail

This fault can be identified in the middle of the signal range. A diagnostic approach that checks only for near-zero or supply voltage can therefore miss the actual criterion. The table does not establish that every P27EB represents an externally open wire despite the generic title.

The internal electronic range-selection configuration must be present, battery voltage must remain above 9 V for more than one second and the monitor must be enabled. Failure is evaluated for 0.100 seconds within a 0.163-second sample window, updated every 6.25 ms. Reporting is Type A, one trip.

Making the reading useful

Identify the raw mode-valve A signal and compare it with the specified forbidden band using the exact service procedure. A driver range request or dashboard indication is different information from this sensor voltage. Verification requires a completed circuit-range assessment, not simply a successful selector movement. Because range and park functions affect vehicle restraint, testing should follow professional procedures that secure the vehicle; the summary supplies no override or disassembly instructions.

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 2280.