OBD-II code · Powertrain (generic, extended)
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.
Is it safe to drive with P21CA?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyModule supply pin fault
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)
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–4 h
- Emissions test
- Likely to fail
- Parts involved
- Scr Control Module
Related codes
- P21C7Reductant Control Module Power Relay Control Circuit/Open
- P21C8Reductant Control Module Power Relay Control Circuit Low
- P21C9Reductant Control Module Power Relay Control Circuit High
- P21CBReductant Control Module Supply Voltage Low
- P21CCReductant Control Module Supply Voltage High
- P21CDReductant Quality Module Supply Voltage Low
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.