What does code P21C4 mean?

Q: What does code P21C4 mean?

Reductant Heater Relay Control Circuit High. P21C4 means the engine control module (ECM) has detected an abnormally high voltage on the control circuit for the reductant heater relay, which is part of the diesel exhaust fluid (DEF) system. The ECM expects the relay control wire to pull to a specific voltage when commanded on or off, and a high reading tells it the circuit is open, shorted to power, or the relay coil is not loading the driver as designed.

DTC codes / P21xx · Fuel and air metering / P21C4

P21C4Reductant Heater Relay Control Circuit High

P21C4 means the engine control module (ECM) has detected an abnormally high voltage on the control circuit for the reductant heater relay, which is part of the diesel exhaust fluid (DEF) system. The ECM expects the relay control wire to pull to a specific voltage when commanded on or off, and a high reading tells it the circuit is open, shorted to power, or the relay coil is not loading the driver as designed.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.