P0641: reference A can fail through instability as well as level

P0641 monitors sensor-reference voltage, but its exact implementation differs between the cited ECM and TCM. The ECM calls it 5-volt reference circuit 1 and checks low level, high level and excessive difference between filtered and unfiltered measurements.

A steady snapshot can miss the fault

The ECM boundaries are 88.64 and 93.18 percent of its reference scale, where 100 percent corresponds to 5.5 volts. Its instability boundary is 0.90 percentage point. These are scaled measurements; 93.18 percent is not a voltage value and does not mean the nominal reference should be 93 percent of five volts.

The TCM has separate bounds, 87.75 and 92.25 percent, and a slightly different filtered-difference limit. Its sample and continuous-failure timing also differ. Do not borrow the ECM calibration to judge a transmission-controller P0641.

Locate the affected reference logically

The code identifies a reference circuit, not a particular attached sensor. Determine the storing controller and consult its actual circuit mapping before associating another sensor fault with this supply. The tables do not list the connected devices.

Verification should assess both reference level and stability while the relevant diagnostic is enabled. A single acceptable average cannot establish that the transient comparison passes. Use the vehicle-specific electrical procedure to resolve the cause; the summary provides no basis for disconnecting guessed components or automatically replacing the controller.

Source and applicability

Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 413. GM 2025-03a, page 1089.