P21CA
Reductant Control Module Supply Voltage Circuit
Internal monitoring of the SCR control module reports a fault on its main supply voltage rail — separate from the relay-control diagnostics, this is the module's own self-diagnosis of its supply pin.
- 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.
- Module supply pin faultCommon
Parts involved
- SCR Control Module
P21CA 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.