P205F
Reductant System Performance (Bank 2)
The DEF system as a whole on bank 2 is not delivering the required NOx reduction — NOx-out remains too high relative to commanded dosing across a drive cycle. Causes range from poor DEF quality, restricted dosing line, weak pump, to a degraded SCR catalyst.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
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.
- Poor DEF quality — diluted or contaminatedCommon
- SCR catalyst thermally agedPossible
- Crystallisation restricts dosing linePossible
Parts involved
- SCR Catalyst
- Reductant Metering Unit
- Reductant Quality Sensor
- Reductant Pump
P205F 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.