P205D: Why a high DEF temperature-sensor voltage represents a cold extreme

P205D reports a high signal voltage from the reductant tank temperature sensor in the cited calibration. Because this sensor is a thermistor, a higher electrical signal corresponds to a lower inferred temperature. “High” in the code label does not mean that the DEF is overheating.

The electrical boundary and temperature interpretation

The table gives a threshold above 4.75 V and annotates that boundary with −40 °C. It describes the check as detecting a signal shorted to power. The monitor accumulates fifty samples at a 0.01-second time base, corresponding to a half-second failure period.

The annotated temperature should be read as part of this input conversion, rather than independent proof that the fluid really reached that temperature. A very cold displayed value accompanied by an electrical fault cannot be assumed to be a trustworthy thermal measurement.

What needs to be established next

Preserve both the raw or available sensor-voltage information and the converted tank temperature. Their relationship is more useful than a temperature screenshot alone. The source requires active vehicle power mode and excludes specified internal controller faults.

The code supports investigating the high electrical input; it does not locate the fault within the sensor, wiring or controller. Before temperature-dependent DEF behavior is interpreted, the validity of this measurement needs to be established through the applicable circuit procedure. These calibration figures must not be transferred automatically to another sensor design or another vehicle that uses the same generic code.

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.