What does code P21C3 mean?

Q: What does code P21C3 mean?

Reductant Heater Relay Control Circuit Low. P21C3 indicates that the powertrain control module (PCM) has detected a low-voltage condition on the control circuit for the reductant heater relay, which is part of the diesel exhaust fluid (DEF) system. The PCM commands the relay to energize the heater, but the feedback signal on the control circuit remains below the expected threshold, suggesting a short to ground, an open circuit, or a faulty relay.

DTC codes / P21xx · Fuel and air metering / P21C3

P21C3Reductant Heater Relay Control Circuit Low

P21C3 indicates that the powertrain control module (PCM) has detected a low-voltage condition on the control circuit for the reductant heater relay, which is part of the diesel exhaust fluid (DEF) system. The PCM commands the relay to energize the heater, but the feedback signal on the control circuit remains below the expected threshold, suggesting a short to ground, an open circuit, or a faulty relay.

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.