P204E: How cumulative pressure movement exposes an erratic DEF signal

P204E evaluates how erratic the reductant pressure signal becomes over a moving window. The monitor does not simply compare the highest and lowest pressure values. It sums the absolute change between measurements, so repeated up-and-down movement can accumulate substantial variation even when the endpoints are similar.

Pump mode changes the allowable movement

In this GM calibration, the moving window contains 100 samples at 0.01-second intervals. The reported limits differ by mode: forward pumping allows more accumulated variation than stopping or heating, and reverse operation uses the tightest listed limit.

This makes pump mode essential diagnostic context. A trace cannot be assessed against one universal “maximum pressure fluctuation” number. The source also says the accumulated string length resets when pump modes switch, which affects how a short section of recorded data should be interpreted.

Check signal validity before diagnosing fluctuation

The test requires a short initialization delay and excludes pressure-sensor low/high electrical faults. A signal that is already electrically invalid is not suitable evidence for the erratic-signal calculation.

For verification, capture sufficiently frequent pressure data, the active pump mode and mode transitions. A slowly refreshed scan display may conceal the repeated variations that contribute to the monitor, so its apparently steady reading is not proof that the calculation passed. The summary identifies abnormal signal behavior; it does not establish whether the origin is electrical noise, sensing failure or genuine hydraulic fluctuation, nor does it prescribe pump 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 188.