P204D

Pressure Sensor Circuit High

P204D is a generic OBD-II powertrain diagnostic trouble code: Pressure Sensor Circuit High. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P204D
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P204D means

P204D is set when the ECM or SCR dosing controller detects that the air pressure in the reductant injection air supply circuit is above the maximum calibrated threshold. On air-assisted DEF injector systems, air pressure is precisely regulated to produce the correct spray pattern; excessive pressure over-atomizes or misdirects the spray, degrading NOx conversion efficiency and potentially damaging the injector tip.

The dosing controller monitors air pressure via a sensor in the supply line. When the measured value exceeds the high-pressure limit during a dosing event or system self-test, this fault is stored. Possible causes include a stuck-open pressure regulator, a faulty pressure sensor reading high, or a system configuration error after component replacement.

The dosing system may suspend or reduce injection events while this fault is active to protect the SCR catalyst and injector. This results in elevated NOx output and, if the derate threshold is reached, a possible torque or speed limitation. The fault should be addressed promptly to restore proper SCR function.

Common causes

Most-frequently reported root causes when P204D is logged.

  • 1
    Failed or stuck-open pressure regulator in the DEF air supply system.
  • 2
    Faulty air pressure sensor providing an erroneously high reading.
  • 3
    Air supply line restriction downstream creating backpressure that reads high at the sensor.
  • 4
    DEF dosing module malfunction delivering unregulated air pressure.
  • 5
    Incorrect pressure regulator installed during a previous service.
  • 6
    Wiring fault causing sensor output to be biased high.
  • 7
    Contamination in the pressure regulator preventing proper pressure relief.

Symptoms drivers notice

MIL illuminated with possible SCR or DEF warning indicator.
Possible suspended or altered reductant dosing.
Elevated NOx emissions.
Possible secondary NOx efficiency DTC.
Potential drivability derate if dosing is suspended and NOx limits are exceeded.

How to diagnose P204D

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and identify companion SCR efficiency or NOx sensor faults.
  2. 2
    Check live air pressure data from the dosing system pressure sensor during system operation.
  3. 3
    Inspect the pressure regulator for proper operation and compare set pressure to specification.
  4. 4
    Check the air pressure sensor calibration by comparing its reading to a known reference gauge if accessible.
  5. 5
    Inspect the air supply line downstream of the sensor for kinks or restrictions causing backpressure.
  6. 6
    Replace the pressure regulator if it is found to be stuck open or out of specification.
  7. 7
    Replace the air pressure sensor if hardware and lines check normal but the sensor continues to read high.

Related powertrain codes

Frequently asked questions

How does P204D differ from P2BAF?

P2BAF indicates air pressure is too low (insufficient for dosing), while P204D indicates air pressure is too high (exceeding the calibrated maximum), suggesting opposite root causes.

Can P204D damage the DEF injector?

Excessive air pressure can erode the injector tip or cause spray pattern distortion over time, so the fault should be corrected promptly.

Is the pressure regulator part of the DEF pump module?

On many systems yes -- the regulator is integrated into the DEF dosing module assembly, requiring module replacement if the regulator fails.

Will clearing P204D restore normal dosing?

Only if the fault was intermittent; if the root cause (stuck regulator or bad sensor) persists, the code will return and dosing may remain restricted.

Disabling P204D in software

RaceTune can permanently disable P204D — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P204D disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch MD1CP004 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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