P21C4Reductant Heater Relay Control Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21C4 means the control side of the DEF heater relay shows voltage when it should be dead. Ford's 2023 diesel summary sets it when the relay stage and all heaters are off and the heater circuit still carries more than 5 V, within at most 5 s; a short to the battery feed or contacts that no longer open are the classic reasons. FCA logs the code on Ram 2500 to 5500 trucks with the 6.7 L Cummins (2019 to 2024) with a title that names the line heater relay, often together with U11C1. Bulletin 25-008-24 traces this pairing to a replacement dosing control unit (68458603AB) and fixes it by reflashing; Stellantis case S1625000002 adds that new units ship without vehicle software and need programming. P21C2 and P21C3 cover the open and low faults.
- What you may notice
- Check engine light on
- Code appears shortly after a DEF control unit was replaced
- Additional communication code such as U11C1 stored
- Likely causes
- LikelyReplacement dosing control unit not programmed or with outdated software (FCA)
- PossibleControl or heater wire shorted to battery voltage
- PossibleRelay contacts welded closed
- RareInternal fault in the controlling module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €0 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.