Powertrain, generic (extended)

P21C4 Reductant Heater Relay Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Replacement dosing control unit not programmed or with outdated software (FCA)
Control or heater wire shorted to battery voltage
Relay contacts welded closed
Internal fault in the controlling module

Symptoms

  • Check engine light on
  • Code appears shortly after a DEF control unit was replaced
  • Additional communication code such as U11C1 stored

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–500
Time0.2–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant heater relay

Related codes

P21C2P21C3P21C7P20BC

References: fordservicecontent.com, static.nhtsa.gov, static.nhtsa.gov.