OBD-II code · Powertrain (generic, extended)
P21C2 Reductant Heater Relay Control Circuit/Open
The control circuit for the relay that powers the DEF heaters is open — main DEF heating supply path interrupted.
Is it safe to drive with P21C2?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyRelay coil open
- Less likelyPin pushed back out of connector body
- Less likelyCracked solder joint inside the module or sensor
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
- Often, with basic tools
- Typical labour
- 0.5–2 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Heater Relay
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.