P2138: Accelerator pedal channels no longer agree

P2138 is a disagreement between the two accelerator pedal position channels in the reviewed GM monitor. The dictionary's D/E sensor terminology maps here to APP1 and APP2. It is not the throttle-position correlation monitor, even though both systems use redundant position information.

Why two normal-looking voltages can still fail

The controller compares displaced pedal positions against a tolerance that increases from 5% near minimum pedal to 10.001% near maximum pedal. It separately compares normalized minimum signals against a 5% reference-scale difference. Two voltages that each remain within their own circuit limits can therefore still represent inconsistent pedal positions.

The individual APP faults P2122, P2123, P2127 and P2128 must be absent for this diagnostic. Reference faults P06A3 and P0697 also inhibit it, and run/crank voltage must exceed 6.41 V. Those prerequisites distinguish a true correlation evaluation from a situation in which one input is already electrically invalid.

Intermittent evidence matters

The listed decision windows are 19 failing counts out of 39 for intermittent behavior or 15 continuous counts. Each count represents 12.5 milliseconds. A brief relationship error can be significant even if the signals look plausible when viewed separately after the event.

For verification, preserve both channels on the same time base and use their correct scaling. Checking APP1 and APP2 at unrelated moments loses the relationship the controller evaluates. P2138 establishes disagreement; the source does not identify the defective channel or prescribe pedal replacement.

Source and applicability

The thresholds and component mapping apply only to the documented monitor, pending vehicle and calibration confirmation. GM diagnostic table, PDF page 602.