Diagnostic Aids

WARNING: This page is about a different car, the 1992 Geo Prizm. However, it is still accessible from the selected car via links, so may be relevant.

Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 will be set in one minute if sensor signal voltage remains between 0-1 volt. Verify clean, tight connection on Brown wire. An open Black or Brown wire, will result in Code 21. If code is intermittent, see TESTS W/O CODES article in this section.

Fig 1: Code 21 Schematic (Except GSi), Oxygen Sensor Circuit Open Or Shorted Circuit

Fig 2: Code 21 Flow Chart (Except GSi), Oxygen Sensor Circuit Open Or Shorted Circuit