P0651: reference circuit 2 has a separate stability monitor

P0651 is the 5-volt reference circuit 2 diagnostic in the cited ECM. Although the generic dictionary calls it reference B circuit/open, the documented test also detects a reference that is too low, too high or changing excessively relative to its filtered value.

Interpret the scaling before the voltage

The low and high boundaries are 88.64 and 93.18 percent on a scale where 100 percent represents 5.5 volts. The filtered-versus-unfiltered difference threshold is 0.90 percentage point. A percentage in this table is therefore neither a direct meter reading nor a percentage of an assumed five-volt full scale.

The decision uses nineteen failed samples in thirty-nine, or 187.5 milliseconds continuously, at a 12.5-millisecond count rate. A stable average can hide the short fluctuations that the second comparison detects.

Conditions select the useful evidence

The supply prerequisite changes with starter state: the table distinguishes a non-engaged starter branch above 6.41 volts with a twenty-five-second condition from an engaged-starter branch above 8.41 volts. Preserve that context when judging a start-related reference complaint.

This code points to reference circuit 2, not a specific sensor attached to it. Use the exact vehicle's wiring and module mapping before assigning another input fault to the same reference. Verification should establish correct level and stability with the monitor enabled. The source does not supply a component list, connector location or automatic replacement instruction.

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 415.