Diagnostic article Generic OBD-II
P204D: high reductant pressure-sensor voltage can display zero pressure
P204D detects excessive reductant pressure-sensor voltage even when the controller substitutes a zero-pressure value.
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Documented application
Documented 2025 GM diagnostic parameter monitor in 25OBDG04B.pdf; model, engine and calibration applicability require vehicle-specific confirmation.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P204D is a high electrical-signal fault for the reductant pressure sensor. In the reviewed GM table, the sensor is analog and the diagnostic looks for output above 4.90 volts. The pressure value shown by a scan tool can be misleading during that fault.
A substituted value changes interpretation
The source states that pressure saturates at 900 kPa between 4.5 and 4.75 volts, then is set to zero above 4.75 volts. Consequently, a zero-pressure display can coexist with a high-voltage circuit fault. It should not automatically be interpreted as proof that the pump is unable to build pressure.
This is a useful distinction from a genuine pressure-performance diagnosis. P204D's direct evidence is excessive sensor voltage and a short-to-power interpretation, not measured hydraulic overpressure.
Conditions and confirmation
The vehicle must be in an active power mode, including accessory, run, start request or propulsion, and specified reductant-controller internal faults must be absent. The row lists six failures within 100 samples at 0.01 second per sample, a nominal 0.1-second requirement and Type A, one-trip reporting.
Verification should compare sensor electrical validity with the displayed pressure value and establish that substitution is no longer active. A normal-looking zero or a changing pump command alone does not answer that question. The table provides no component location, pinout or repair recommendation. Its thresholds and substitution behavior are limited to this documented 2025 GM reductant-control implementation until the vehicle application is confirmed.
Source: GM monitor, PDF page 188.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Reductant Pressure Sensor Circuit High
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT