Code 21 - Main O2 Sensor CKT (Federal - Front O2 Sensor)
WARNING: This page is about a different car, the 1995 Lexus GS 300. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
After repairs are completed, perform TROUBLE CODE DETECTION DRIVING PATTERN TEST.
Fig 1: Code 21 Front Oxygen Sensor Flow Chart (GS300 Federal) Main O2 Sensor Circuit (Front O2 Sensor)