What does code P21C9 mean?

Q: What does code P21C9 mean?

Reductant Control Module Power Relay Control Circuit High. P21C9 indicates that the reductant control module has detected a high voltage condition on the power relay control circuit, meaning the relay coil control wire is seeing voltage above the expected range. The module monitors this circuit to ensure the relay can be commanded on and off properly for diesel exhaust fluid (DEF) system operation.

DTC codes / P21xx · Fuel and air metering / P21C9

P21C9Reductant Control Module Power Relay Control Circuit High

P21C9 indicates that the reductant control module has detected a high voltage condition on the power relay control circuit, meaning the relay coil control wire is seeing voltage above the expected range. The module monitors this circuit to ensure the relay can be commanded on and off properly for diesel exhaust fluid (DEF) system operation.

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.