Possible causes

– Faulty Coolant Flow Control Valve Position Sensor
– Coolant Flow Control Valve Position Sensor harness is open or shorted
– Coolant Flow Control Valve Position Sensor circuit poor electrical connection

Tech notes

There is a Toyota factory service bulletin for the following Toyota models:

2004-2007 Toyota Prius
Toyota Factory Service Bulletin OBDII Code P1121

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

TOYOTA Technical Service Bulletins for P1121 code