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.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Scr catalyst
- Reductant metering unit
- Reductant quality sensor
- Reductant pump
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Poor DEF quality — diluted or contaminated
- SCR catalyst thermally aged
- Crystallisation restricts dosing line
Possible 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)
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
- Reduced-power or limp mode may occur
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.