B121D Code: Meaning, Causes, and Symptoms | OBD2 Codes

B121D – Rear HVAC Mode Control Input Circuit Short

 

B121D Possible Symptoms

 

Engine Light ON (or Service Engine Soon Warning Light)

 

B121D Possible Causes

 

Faulty Rear HVAC Mode Control,Rear HVAC Mode Control harness is open or shorted,Rear HVAC Mode Control circuit poor electrical connection,Faulty Heating, Ventilation, and Air-Conditioning (HVAC) Module

 

B121D Description

 

Under normal operation, the rear auxiliary control is supplied a 5-volt reference and ground from the Heating, Ventilation, and Air-Conditioning (HVAC) Module. When the auxiliary blower motor speed, airflow mode or temperature control is adjusted, the HVAC module reads the voltage from the rear auxiliary climate controls on the auxiliary blower motor speed, airflow mode, and temperature control feedback circuits to determine the requested setting. The OBDII code sets when the HVAC module senses an open, or short to ground or voltage on the rear auxiliary climate controls temperature input circuit.