What does code P21CB mean?

Q: What does code P21CB mean?

Reductant Control Module Supply Voltage Low. P21CB indicates that the reductant control module, which manages the diesel exhaust fluid (DEF) system, has detected its supply voltage is lower than the manufacturer's specified threshold. The module monitors the voltage feeding its internal circuitry and sets this code when the supply drops below a safe operating level, potentially disrupting DEF dosing and emissions compliance.

DTC codes / P21xx · Fuel and air metering / P21CB

P21CBReductant Control Module Supply Voltage Low

P21CB indicates that the reductant control module, which manages the diesel exhaust fluid (DEF) system, has detected its supply voltage is lower than the manufacturer's specified threshold. The module monitors the voltage feeding its internal circuitry and sets this code when the supply drops below a safe operating level, potentially disrupting DEF dosing and emissions compliance.

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.