P20C9
Reductant Control Module Requested MIL Illumination
The dedicated reductant/SCR control module has requested that the master PCM illuminate the malfunction indicator lamp — a generic forward of an internal SCR fault. Read the SCR module subcodes for the actual cause.
- 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.
- Internal SCR module fault — read subcodesCommon
Parts involved
- SCR Control Module
P20C9 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.