Diagnostic Aids

Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 will be set in one minute on a restart if a fault is present in either the O2 signal voltage line or in the heater HT terminal.

Verify a clean and tight connection ECM ground connection. An open or shorted Black/Red or Black wire will set Code 21. If code is intermittent, see TESTS W/O CODES article in this section.

Fig 1: Code 21 Schematic (Calif. GSi), Oxygen Sensor Circuit Open Or Short Circuit

Fig 2: Code 21 Flow Chart (Calif. GSi), Oxygen Sensor Circuit Open Or Short Circuit