P205E: Recognizing an erratic DEF tank temperature signal

P205E in this GM monitor is an erratic-temperature diagnosis, not simply a temperature that is too hot or too cold. The controller checks whether the tank temperature signal changes too much within a short sequence of measurements.

Small reversals can accumulate into a failure

The documented window contains ten samples taken at 0.1-second intervals. Its “string length” is the sum of the absolute changes, with a boundary above 4 °C per window. Temperature that rises and falls repeatedly can exceed that accumulated limit even if its first and last values are close.

The failure decision uses repeated sample results rather than one isolated endpoint difference. Looking only at a one-second start-to-finish change can therefore miss the behavior the controller is evaluating.

Separate erratic data from an invalid circuit

The monitor excludes low/high temperature-sensor electrical faults and specified sensor power-supply faults. Those exclusions matter because a signal outside its electrical operating range should not be interpreted as an ordinary fluctuating temperature measurement.

A useful diagnostic capture includes the sequence of temperatures at adequate resolution, sensor supply status and the active fault list. A smooth-looking low-refresh display cannot independently demonstrate that the original signal was stable. Conversely, a large legitimate temperature difference between distant observations is not the same as rapid variation within this window. The source supports investigating unstable information; it supplies neither a tank-removal procedure nor evidence that the temperature sensor is always the failed part.

Source and scope

2025 GM 25OBDG04B DEFC monitor calibration only; this document does not establish applicability to every model, engine or reductant system. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 190.