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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P204D

P204DReductant Pressure Sensor Circuit High

P204D means the engine control module has detected a voltage signal from the reductant pressure sensor that is higher than the expected range for the current operating conditions. The sensor monitors the pressure of the diesel exhaust fluid, known as reductant, in the selective catalytic reduction system, and a high signal typically indicates an open circuit, a short to voltage, or a sensor that is reading above the plausible maximum.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • A message on the driver information display warning of low reductant pressure or a reductant system fault
  • Reduced engine power or limp mode triggered by the control module as a protective strategy
  • The vehicle may refuse to restart after the engine is shut off once the fault is active
  • A noticeable increase in diesel exhaust fluid consumption in some cases
  • Rough running or poor acceleration if the system enters a derate strategy

Common causes, most likely first

  1. An open circuit or high resistance in the signal wire between the reductant pressure sensor and the control module
  2. A short to voltage on the sensor signal circuit, often from chafed wiring near the exhaust or reductant tank
  3. A failed reductant pressure sensor that is stuck at a high voltage output
  4. Corroded or damaged connector pins at the sensor or the control module
  5. A faulty control module driver for the sensor circuit, though this is less common
  6. Contamination or debris in the reductant pressure sensor port causing an abnormal reading
  7. A poor ground connection for the sensor, which can skew the signal voltage high

How to diagnose and fix it

  1. Connect a scan tool and record the freeze-frame data for P204D, then clear the code and verify whether it returns immediately or only under specific conditions
  2. Visually inspect the reductant pressure sensor, its connector, and the wiring harness for damage, corrosion, or chafing, especially near hot exhaust components
  3. Measure the sensor reference voltage and ground at the connector with a multimeter, comparing the readings to the manufacturer's specification
  4. Check the signal wire for continuity to the control module and for a short to voltage or ground using a wiring diagram
  5. If the wiring and connectors are intact, substitute a known-good sensor or perform a resistance and voltage output test on the existing sensor
  6. Verify the repair by clearing the codes, running the reductant system monitor, and confirming that the sensor voltage stays within the normal range during a road test
  7. If the fault persists after all external checks, follow the manufacturer's diagnostic flow for control module internal circuit testing before replacing any module

High signal often means open circuit

  • A high signal code is frequently caused by an open circuit or a disconnected sensor rather than a sensor that has failed internally, so resistance checks on the wiring are more valuable than swapping parts.
  • Technicians sometimes replace the reductant pressure sensor only to find the real fault is a chafed signal wire near the exhaust hanger or a corroded connector pin that was missed during a quick visual inspection.
  • The control module may set P204D as a companion code when the reductant pump or dosing module is commanded off, so always check for related pressure or pump codes before condemning the sensor.
  • A weak or missing ground at the sensor can pull the signal voltage high, and this is easily overlooked when the ground circuit is shared with other sensors on the same harness.
  • Clearing the code without repairing the root cause often results in the fault returning after a few drive cycles, and some vehicles will enter a countdown-to-restart-inhibit strategy that makes the vehicle undrivable.

Can I keep driving with P204D?

You can usually drive the vehicle for a short distance to a repair shop, but avoid long trips and heavy loads because the control module may derate engine power or prevent a restart once the fault is active. Do not ignore the warning light, as continued driving with an active reductant system fault can lead to a no-start condition.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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