P206D: A DEF quality reading above range is not a direct voltage measurement

Although the generic P206D description says circuit high, this GM implementation evaluates a quality value received from the smart UTLC sensor. The reductant controller checks whether the reported measurement exceeds its range while the sensor says that measurement is ready.

The two inputs belong together

The documented criterion combines quality above 63.25 percent with a true quality-readiness bit. The controller receives both over the sensor's communication path. It is therefore incorrect to reinterpret 63.25 percent as a connector voltage or as a universal concentration specification for acceptable DEF.

The row specifies a two-second failure period made from four half-second samples. An unavailable quality measurement and a ready but excessive measurement are separate conditions; the readiness bit establishes which interpretation is applicable.

What the result can and cannot establish

SENT communication faults and internal controller faults are excluded. If the data path is unreliable, an implausible percentage cannot be evaluated as though it were an independent laboratory analysis of the fluid.

Useful evidence consists of the reported percentage, readiness state, persistence and communication health. These help determine whether the monitor actually observed an out-of-range quality report. They do not establish the chemical cause or justify adding an unspecified substance to the tank. Any fluid assessment or sensor-circuit diagnosis needs the vehicle's applicable service information. The source describes a smart-sensor range check, not a complete contamination diagnosis.

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 192.