P205F: Reductant System Performance (Bank 2)
Standardized SAE J2012 code · P20xx — Fuel & air metering and auxiliary emission controls
What does P205F mean?
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.
The code tells you which monitor failed, not which part is broken. Test before replacing parts.
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
- High Poor DEF quality — diluted or contaminated
- Medium SCR catalyst thermally aged
- Medium Crystallisation restricts dosing line
Likelihood is a general estimate across vehicles; your make and model may differ.
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