P203B: When the DEF level sensor cannot provide a trustworthy measurement

P203B in this GM DEFC calibration checks whether the ultrasonic reductant level sensor can provide usable measurements when conditions should allow it. The important input is the sensor's readiness bit, which describes confidence in the returning acoustic echo.

An unreadable level is different from an empty tank

The failure criterion is a false readiness bit. The monitor is enabled only with estimated DEF volume above five litres, suitable tank and sensor temperatures, and more than 300 seconds since refill or draining. It also requires specified motion and agitation conditions while excluding slosh detection and selected heater operation.

Those conditions explain why an isolated unavailable measurement just after filling, or under unsuitable thermal conditions, cannot be judged as the same event. The controller deliberately waits for circumstances in which a reliable echo is expected.

Evidence that changes the next decision

First distinguish an absent trustworthy reading from a valid reading that reports an abnormal height. P203B concerns the former. Second, establish whether level voltage, SENT communication or internal controller faults prevented the monitor from operating; these are explicit exclusions.

The documented failure accumulation can extend to 200 seconds, so a short observation window may not reproduce it. A useful diagnostic record includes readiness, estimated quantity, refill timing, temperature, movement and the relevant inhibiting faults. These data support deciding whether the sensing conditions were valid. They do not, by themselves, establish that the complete tank assembly requires replacement.

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 186.