P0561: compare the TCM battery feed with its ignition feed

P0561 is labelled system-voltage instability in many code lists. The cited GM transmission implementation is more specific: it compares the TCM's battery-voltage reading with its ignition-voltage reading. A difference between those feeds can be present even when a separate measurement elsewhere on the vehicle looks acceptable.

The comparison has a persistence requirement

The documented failure criterion is an absolute difference of at least 3 V. It must occur in 40 of 50 samples, using a 100-millisecond loop. This is a repeated disagreement between two controller inputs, not simply one low battery reading or a requirement to measure alternator ripple.

Evaluation also depends on a calibration enabling this diagnostic and on the TCM actually having the battery-voltage circuit being compared. Those hardware and configuration requirements matter before applying the monitor to a different transmission controller.

Evidence that narrows the investigation

Record both supply readings together and preserve their difference during the original event. Comparing readings taken at different times can conceal a temporary feed problem. The table also describes ignition-voltage hysteresis and excludes certain clearing or end-of-trip processing states, so a disabled monitor should not be mistaken for a passing result.

The source does not assign the difference to an alternator, ignition switch or TCM replacement. Each supply path must be investigated using the correct wiring and service information. After any repair, the meaningful result is consistent feed behavior while the comparison is enabled, with the original symptoms and associated electrical faults also evaluated.

Source and applicability

GM 2015 15OBDG08B transmission-control summary. Only the cited monitor and calibration are described; per-code vehicle and engine applicability has not been established.

15OBDG08B Transmission Diagnostics, physical PDF page 4.