P056D
Reductant Control Module System Voltage
The reductant (AdBlue) control module reports its supply voltage outside the calibrated window. Reductant dosing is suspended until voltage returns to range, which can drive NOx emissions out of compliance.
- 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.
- Supply voltage lowCommon
Parts involved
- Reductant Control Module
- Reductant Supply Relay
P056D 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.