What it means
DEF pressure sensor reads within electrical range but plausibility fails — does not rise during prime, drift over a drive cycle, or stuck at a constant value.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant pressure sensor reports the pressure of DEF/AdBlue delivered by the SCR dosing system. The control module compares its signal with expected pressure during pump priming and operation; P204B sets when the signal remains electrically plausible but does not track the expected pressure rise or change over time.
Common causes
Most likely first.
- Sensor drift or stuck reading
- Pump pressure builds slowly
- Sensor element aged beyond calibration window
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and check live reductant pressure during a commanded prime to see whether the reading rises and changes plausibly.
- Inspect the pressure sensor connector and nearby harness for corrosion, loose pins, fluid intrusion, or damage that could leave the signal stable or biased.
- Compare the sensor reading with the system's specified pressure response during priming and, if available, compare it with an independent pressure measurement using the approved procedure.
- Check the DEF supply, filter, and pump operation for restrictions or slow pressure build that could make a sound sensor appear unresponsive.
- If pressure builds correctly but the scan reading stays fixed or drifts, test the sensor signal and its reference and ground circuits to the manufacturer's specification.
- Replace or further test the pressure sensor only after wiring and actual pressure response have been assessed.
Common fixes
- Repair a loose or damaged pressure-sensor connector or harness section.
- Correct a DEF supply restriction or pump issue if measured pressure builds too slowly.
- Replace the reductant pressure sensor if its signal is stuck or drifts despite correct pressure and wiring.
- Clear the code and verify that pressure tracks the priming and operating sequence.
Can I keep driving?
If the vehicle runs normally, a short trip may be possible, but arrange diagnosis promptly and avoid relying on it for long journeys. The SCR warning may progress to a no-start countdown, and emissions performance or inspection eligibility can be affected.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.