OBD-II code · Powertrain (generic, extended)
P21CB Reductant Control Module Supply Voltage Low
SCR control module supply voltage measured at the module pin is below the lower threshold.
Is it safe to drive with P21CB?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyVoltage drop in supply harness
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
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–3 h
- Emissions test
- Likely to fail
- Parts involved
- Scr Control Module
Related codes
- P21CAReductant Control Module Supply Voltage Circuit
- P21C8Reductant Control Module Power Relay Control Circuit Low
- P21C9Reductant Control Module Power Relay Control Circuit High
- P21CCReductant Control Module Supply Voltage High
- P21CDReductant Quality Module Supply Voltage Low
- P21CEReductant Quality Module Performance
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.