P27EF: An invalid middle-band voltage can produce a circuit-open result

P27EF identifies an invalid voltage band on the transmission mode-valve B position-sensor circuit. In the documented internal electronic range-selection system, the suspicious value is between two boundaries near the middle of the signal range. The generic circuit/open wording therefore should not be reduced to an assumption of zero voltage.

Recognize the invalid band

The specified condition is raw voltage above 1.263 V and below 1.504 V. Separate low- and high-circuit monitors use different boundaries. A signal can consequently be neither at the low fault limit nor at the high fault limit and still be electrically invalid for this position encoding.

The monitor requires battery voltage above 8 V for more than one second, diagnostic enablement and the relevant internal range-selection configuration. Its source scope specifically names GF9 and CVT applications; this does not establish fitment for every transmission reporting the generic code.

Preserve raw voltage and configuration

The table requires 0.100 second within a 0.163-second sample period, using a 6.25 ms update. One transient crossing of the middle band during a change cannot be assumed to equal that confirmed result.

Useful evidence includes the raw sensor voltage, its persistence and the actual system configuration. A decoded range label alone hides the electrical band that caused the classification. The monitor does not establish that the valve is mechanically open or identify where a circuit interruption is located.

Source and applicability

2025 GM 25OBDG03B documented monitor only. Vehicle, engine and calibration applicability must be confirmed from the matching service information; no model fitment is inferred. These criteria explain controller logic, not a complete repair procedure. 25OBDG03B, PDF page 1697. 25OBDG03B, PDF page 1703. 25OBDG03B, PDF page 1704.