P204B
Reductant Pressure Sensor Circuit Range/Performance
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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
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)
Common causes
Ordered by how often each turns out to be the cause.
- Sensor drift or stuck readingCommon
- Pump pressure builds slowlyPossible
- Sensor element aged beyond calibration windowLess likely
Parts involved
- Reductant Pressure Sensor
P204B is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.