P204C: Why a zero DEF pressure reading can hide an electrical low signal

P204C is a low electrical signal diagnosis for the reductant pressure sensor in this GM DEFC table. The documented failure boundary is a sensor output below 0.45 V. The source associates this with a short to ground or an open circuit, but the code does not distinguish those possibilities by itself.

The displayed pressure can be misleading

The table explains how the controller converts out-of-range voltage. A portion of the low range is saturated at −50 kPa, while voltages below 0.35 V cause the pressure variable to be set to zero. A displayed zero is therefore not necessarily a reliable observation of an unpressurized line.

This is the key reason to retain electrical signal information alongside calculated pressure. Treating a substituted zero as a physically measured value could send the investigation toward pump performance when the input itself has already failed its electrical check.

What constitutes relevant verification

Monitoring requires active vehicle power mode and excludes specified internal controller faults. The table describes sample-based fault accumulation, so a transient value and a matured diagnostic should be distinguished.

The useful next distinction is whether the sensor input is electrically valid before any pressure-control result is interpreted. Vehicle-specific circuit information is needed to separate an open path, a ground fault and sensor-related behavior; no connector pinout is provided by this summary. Pressurized-system disassembly is not necessary to understand what this monitor reports and should not be improvised from the code description.

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.