P21C5: available reductant level data stays too still during expected slosh

P21C5 is a stuck reductant level-sensor diagnosis. The reviewed GM monitor assumes that measurements are available and trusted, then asks whether they change enough when fluid movement is expected. It is different from a false-readiness or electrical-range fault.

Motion and readiness are essential

The controller compares the current level with the reading from 1,000 milliseconds earlier using a 0.3-mm change criterion. Vehicle speed must exceed 5 km/h, its validity must be true, and calculated longitudinal acceleration must exceed 0.1 m/s². The reductant must be liquid and estimated quantity above five liters.

Readiness is filtered: six consecutive true samples establish a ready flag, while one false sample removes it. With a 0.1-second time base, that means 600 milliseconds to establish readiness and 100 milliseconds to lose it. An unavailable measurement is not treated as a stable valid level.

Persistence and verification

The table describes 200 seconds to fail and 10 seconds to pass using an asymmetric step-up/down counter. Relevant level, temperature, CAN, SENT and controller-internal faults are excluded. Reporting is Type B over two trips.

Verification should show ready measurements responding during the qualified motion conditions. A level that stays constant while the vehicle is stationary does not reproduce the test. Conversely, a changing but unready value is not proof of a pass. The source does not establish a specific sensor defect or tank repair; exact vehicle applicability and the appropriate controlled drive procedure require service information.

Source: GM monitor, PDF page 202.