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Test TC-9A - Left O2S Stays At Center (DTC 21)

NOTE: For circuit & connector terminal identification, see CONNECTOR IDENTIFICATION DIRECTORY TABLE  . For wiring diagrams, see WIRING DIAGRAMS  section. After each repair, perform TEST VER-2  .
NOTE: DRB displays left oxygen (O2) sensor as O2 and right oxygen (O2) sensor as O2R.
  1. Start engine. Using DRB, read left O2 sensor voltage. If left O2 sensor voltage is locked on 0.5 volt, go to step 5). If left O2 sensor voltage is not locked on 0.5 volt, go to next step.
  2. Wiggle O2 sensor wiring while observing DRB display. If at any time O2 sensor voltage locks on 0.5 volt when wiring is wiggled, repair harness or connector that caused voltage to lock (open circuit). If left O2 sensor voltage does not lock on 0.5 volt when wiring is wiggled, go to next step.
  3. At this time, the condition required to set trouble code is not present. LEFT O2S STAYS AT CENTER trouble code sets if left O2 sensor output voltage stays at 0.5 volt for 1.5 minutes with engine temperature more than 170°F (76.6°C) and engine running for 2 minutes. Possible causes are: left O2 sensor circuit open or left O2 sensor failure. Go to next step.
  4. Inspect left O2 sensor wiring and connectors. Repair wiring and connectors as required. If wiring and connectors are okay, see INACTIVE TROUBLE CODE CONDITION  in this article. Test is complete.
  5. If left O2 sensor is locked on 0.5 volt in step 1), disconnect left O2 sensor connector. Using DRB in ohmmeter mode, check resistance of O2 sensor connector terminal No. 3 (Black/Light Blue wire). If resistance is more than 5 ohms, repair open Black/Light Blue wire. If resistance is less than 5 ohms, go to next step.
  6. Connect a jumper wire between left O2 sensor connector terminal No. 2 (Black/Dark Green wire) and positive battery terminal. Using DRB, read left O2 sensor voltage. If voltage is more than one volt, replace left O2 sensor. If voltage is less than one volt, go to next step.
  7. Turn ignition off. Disconnect Powertrain Control Module (PCM) connector. PCM is located under air cleaner. Leave jumper wire connected place. Using DRB in voltmeter mode, check voltage on PCM connector terminal No. 41 (Black/Dark Green wire). If voltage is more than 10 volts, replace PCM. If voltage is less than 10 volts, repair open Black/Dark Green wire.
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