Diagnostic Aids

Normal O2 sensor voltage varies between .10-.90 volt. Code 21 sets in one minute if sensor signal voltage remains between 0-1 volt. Verify clean tight connection on Brown wire. An open Black wire will result in Code 21. If code is intermittent, see TESTS W/O CODES article.

Fig 1: Code 21, Circuit Wiring Diagram & Flow Chart

Courtesy of GENERAL MOTORS CORP.