Diagnostic Aids

Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 sets in one minute if sensor signal voltage remains between 0-1 volt. Verify clean tight connection on Black wire (VIN 5) or Brown wire (VIN 6). An open Red or Black wire (VIN 5) or Black or Brown wire (VIN 6), will result in Code 21. If code is intermittent, see TESTS W/O CODES article.

Fig 1: Code 21 VIN 5 Circuit Diagram

Fig 2: Code 21 VIN 6 Circuit Diagram

Fig 3: Code 21 Flow Chart