P2866: When rod 5 feedback stays high instead of pulsing

The P2866 monitor checks the high side of rod 5's digital position feedback. A high circuit report does not, by itself, mean that the shift fork has travelled beyond its mechanical limit. The transmission controller first judges whether the incoming electrical signal is usable.

High level versus high duty cycle

The source identifies a no-pulse condition with pin voltage above 3,500 mV. It also identifies an input duty cycle above 92 percent. A stationary high input and a pulse train dominated by its high portion are therefore separate routes to this code. The failure confirmation is at least 150 milliseconds.

Interpreting a captured signal requires preserving which route failed. A single averaged voltage can hide a duty-cycle problem, while a position value substituted by the controller may conceal loss of the original pulses. These are reasons to compare the signal-validity information with the reported position rather than infer a jammed fork from the code name.

Conditions that affect confirmation

The monitor excludes cranking and the key-stop state and lists battery/reference-supply faults as disabling conditions. Its VREF2 comparison entries appear contradictory, so their numerical values are not presented here as a workshop specification.

A meaningful follow-up confirms that pulse validity returns while the relevant supply diagnostics permit the monitor to operate. The table does not identify a replacement part or authorize internal transmission work. Matching vehicle service information is necessary before electrical access or component decisions.

Source and applicability

2025 GM 07A-2 documented monitor only; module and calibration applicability must be matched to the vehicle. The application-matrix filename join remains unresolved; no model or engine fitment is inferred. GM PDF, page 915.