P204B: The reductant pressure plausibility check before pressurization

P204B concerns the credibility of the reductant pressure signal before the system develops operating pressure. In the cited GM calibration, the check is associated with the pump's startup state and an expectation that the unpressurized system is near ambient pressure.

A startup offset can fail in either direction

The pressure limits are below −36.4 kPa or above +36.4 kPa relative to the monitor's reference. The row also includes a timeout if the internal decision counter fails to reach a pass or fail decision within the specified period. A sensor that stays undecided is therefore not automatically considered healthy.

The strategy permits retries, with a maximum sequence described as approximately 33 seconds. The source places recovery on a subsequent driving cycle. A brief check immediately after a fault is recorded may consequently be insufficient to demonstrate recovery.

Separate zero plausibility from delivery performance

A reading outside this startup window is different from pressure failing to follow a setpoint while fluid is being pumped. The former questions the baseline measurement; the latter evaluates operation under demand. Applying an operating-pressure specification to P204B would address the wrong stage.

Electrical low/high pressure-sensor faults and sensor supply faults inhibit the plausibility assessment. Preserve pump state, pressure, validity and electrical faults together before deciding what the pressure value means. The monitor supports investigating an implausible baseline; it does not identify a leaking line, a weak pump or a defective sensor without additional evidence.

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.