P085F: manual-shift input can be stuck inside a valid electrical range

P085F in the cited GM MGG table checks whether the manual up/down shift input is stuck within a recognized signal range. This differs from P0827 and P0828, where the normalized signal is outside the allowed low or high boundary.

A plausible voltage can still represent a fault

The switch value is calculated relative to ignition voltage. The table describes two in-range bands, each with its own persistence conditions. One branch uses a 30-second interval; another uses a cumulative period across selector-range tests.

Consequently, a meter reading that is neither near ground nor near supply does not establish normal switch behavior. The relevant question is whether the signal changes as the control and selector state require.

Record the sequence rather than only the final value

The controller's decision depends on time and operating state. A short trace of the input alongside the driver's request and selector position is more useful than a single screenshot taken after the control has been released. This is a practical inference from the stuck-in-range design.

The cited rows also qualify supply and, where stated, engine-speed communication. They are special diagnostics without MIL illumination, so the engine lamp is not a pass criterion.

The summary does not establish that the physical switch is mechanically jammed. Wiring, signal interpretation and controller configuration require the matching vehicle procedure. Verify that the input can make the required transitions under valid test conditions before declaring the fault resolved.

Application limits

2015 GM 15OBDG06 TCM MGG unique section. Two P085F in-range branches are documented; exact vehicle mapping and service test remain separate.

Source

15OBDG06 Transmission Diagnostics, physical PDF pages 85.