P203D: Why a DEF level circuit-high label can mean an excessive reported height

The words “circuit high” can suggest a simple analogue voltage measurement, but the documented P203D implementation evaluates a level value supplied by a smart ultrasonic sensor. The sensor sends its information to the reductant controller over SENT.

The high measurement has to be ready

The table associates P203D with a reductant height exceeding 400 millimetres while the level readiness bit is true. This combination matters: an unavailable echo is a different condition from an accepted measurement beyond the stated measuring range.

The failure period is six seconds, represented by twelve half-second samples. The row also identifies a shorter success count. Consequently, the timing of a readiness transition and the persistence of the excessive value are useful when examining recorded data.

Avoid treating a processed signal as a test-pin voltage

The 400-millimetre threshold is a sensor measurement limit in this calibration, not a wiring voltage or a universal maximum fill instruction. The source does not establish whether an excessive report arose from the fluid geometry, sensing hardware or another condition.

Power-supply faults and SENT communication faults are excluded before this monitor can provide its intended result. If they are active, resolving the reliability of the measurement path takes precedence over interpreting the height as genuine. Compare the reported level, readiness and fault state together, using vehicle-specific information to establish the sensor arrangement. The code alone is insufficient evidence for draining fluid or replacing a tank.

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 187.