P2B51: The exhaust profile monitor detects a pin moving without a command

P2B51’s generic description includes performance/stuck-off wording, while this GM monitor describes an exhaust profile-actuator pin firing without being commanded. That difference must remain visible. A failure to move after a request and movement without a request are not the same diagnostic observation.

Read command and movement together

The controller recognizes extending, fully extended and retracted feedback states. The row’s failure occurs when the actuator crosses the extending boundary without a command. A final position reading alone can miss the contradiction if the pin later returns normally.

The printed state boundaries are internal feedback values without a clear physical unit on this page. They are not safe substitutes for a travel specification or connector-voltage test. The supported evidence is the sequence of recognized state changes relative to the controller’s request.

Preserve cylinder-event sampling

The engine must be running and system voltage above 11 V, with the diagnostic enabled. Four incorrect positions among five cylinder-event reads establish the result. The one-trip reporting class does not replace that repeated event-level observation.

Useful investigation retains the exhaust-channel identity and command history as well as feedback. The source names actuator 2 but does not establish a physical-cylinder mapping from the generic letter designation. The code cannot by itself identify whether the mechanism moved unexpectedly or the feedback misrepresented it; that distinction requires the correct service procedure.

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 877.