What does code P21C8 mean?

Q: What does code P21C8 mean?

Reductant Control Module Power Relay Control Circuit Low. P21C8 indicates that the reductant control module has detected a low-voltage condition on the power relay control circuit, meaning the module is not receiving the expected voltage to energize the relay that supplies power to the diesel exhaust fluid (DEF) system. The computer monitors the control circuit and sets this code when the voltage falls below the calibrated threshold, often due to a short to ground, open circuit, or faulty relay.

DTC codes / P21xx · Fuel and air metering / P21C8

P21C8Reductant Control Module Power Relay Control Circuit Low

P21C8 indicates that the reductant control module has detected a low-voltage condition on the power relay control circuit, meaning the module is not receiving the expected voltage to energize the relay that supplies power to the diesel exhaust fluid (DEF) system. The computer monitors the control circuit and sets this code when the voltage falls below the calibrated threshold, often due to a short to ground, open circuit, or 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.