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P21C4 on the 2023 Ram 5500

The bulletins listed name this code in NHTSA's summary, or are about the same components. A bulletin or a recall listed here may apply to the model year. None of it is a diagnosis.

Nobody has written a description of this code yet.

Bulletins for this model year that name P21C4
91005982025-05-29. Service bulletin. Module, Urea Pump/Level: HD 6.7L (ETL/ETN/ETM) with Mil On P21C4-00 (Reductant Line Heater Relay Control Circuit High / U11C1-00 (Reductant Control Module Received Implausible Data From ECM/PCM), do not replace the DEF Pump. Follow TSB's released and appropriate Service Diagnostics.
91005082025-03-28. Service bulletin. "6 CONTROL UNIT, Diesel Exhaust Customer complaints with malfunction light illuminated on 2019 thru 2024 MY DJ / D2 / DD / DP vehicles with 6.7L (ETL / ETM / ETN) built on or before October 15,2024 and exhibiting DTCs P21C4-00 (Reductant Line Heater Relay Control Circuit High) and / or U11C1-00 (Reductant Control Module received implausible Data from ECM / PCM). These DTCs will occur after installation of service DCU 68458603AB. Do not replace parts until after the TSB 25-008-24 is performed. If the DTC(s) continue to set, then follow the Service Diagnostics for the applicable DTC."
S1625000002 REV.B2025-03-13. Service bulletin. Malfunction Indicator Lamp (MIL) Illumination, Diagnostic Trouble Codes (DTCs) P21C4 – (Diesel Exhaust Fluid) Reductant Line Heater Control Circuit High, and U11C1 – Reductant Control Module Received Implausible Data From ECM/PCM, Set
25-008-242024-11-18. Service bulletin. "Flash: Diesel Exhaust Fluid (DEF) Dosing Control Unit (DCU) Update Customers must experience a Malfunction Indicator Lamp (MIL) illumination and the vehicle must exhibit/set the following Diagnostic Trouble Code (DTC): ● P21C4-00 - Reductant Line Heater Relay Control Circuit High. ● U11C1-00 - Reductant Control Module Received Implausible Data From ECM/PCM. These DTCs will occur after installation of service DCU 68458603AB. This bulletin involves reprogramming the DCU with the latest available software."

4 on file from NHTSA, as of 2026-10-19. See them in the bulletin list. A bulletin lists model years: read the document for which vehicles it concerns.