OBD-II code · Powertrain (generic, extended)
P21C9 Reductant Control Module Power Relay Control Circuit High
SCR control module power-relay control line stuck high. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
Is it safe to drive with P21C9?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyShort to B+
- Less likelyConnector pins corroded - high contact resistance
- Less likelySupply wire pinched against B+ source
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
- 0.5–2 h
- Emissions test
- Likely to fail
- Parts involved
- Scr Control Module, Power Relay
Related codes
- P21C7Reductant Control Module Power Relay Control Circuit/Open
- P21C6Reductant Level Sensor B Stuck
- P21C8Reductant Control Module Power Relay Control Circuit Low
- P21CAReductant Control Module Supply Voltage Circuit
- P21CBReductant Control Module Supply Voltage Low
- P21CCReductant Control Module Supply Voltage High
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.