Diagnostic article Generic OBD-II
P27F1: Low range-position feedback can represent more than a ground short
Learn why P27F1’s low-voltage classification does not uniquely identify a grounded wire.
Read before applying this article
Documented application
2025 GM 25OBDG03B documented monitor only. Vehicle, engine and calibration applicability must be confirmed from the matching service information; no model fitment is inferred.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P27F1 monitors raw mode-valve B position-sensor voltage below the valid range. The GM description explicitly associates the low result with a ground short or an open circuit. That distinction prevents the low label from being treated as proof of one specific wiring defect.
Use the electrical value, not a decoded range label
The cited low threshold is below 0.414 V. The same document has a separate invalid middle-band monitor, P27EF, and a high-voltage monitor, P27F2. Those classifications describe different signal conditions; a scan tool’s Park or Drive text does not replace the raw-voltage evidence.
The diagnostic applies to the documented internal electronic range-selection configuration, with battery voltage above 8 V for more than one second. It is not a universal voltage specification for every range switch or transmission family.
Confirm the qualified low interval
The row specifies 0.100 second within a 0.163-second sample period and a 6.25 ms update. A brief power-up or transition value outside the enable window is not necessarily the recorded failure condition.
Retain the raw signal, supply state and configuration when tracing the event. Because both an open path and a grounded path can lead to this classification in the described circuit, a suitable circuit test must distinguish them. The monitor does not directly prove that mode valve B is mechanically stuck closed.
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.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Transmission Range Control B Position Sensor/Switch Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT