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Test FC-8A - Left O2S Stays At Center (DTC 21)

NOTE: For connector terminal identification, see CONNECTOR IDENTIFICATION . For appropriate wiring diagram, see WIRING DIAGRAMS  at end of article.
  1. Start engine. Using DRB-II, 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, wiggle O2 sensor wiring while observing DRB-II display.
  2. 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). Perform TEST VER-2. If 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 fault is not present. LEFT O2S STAYS AT CENTER fault sets if 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: O2 sensor circuit open or O2 sensor failure.
  4. Inspect O2 wiring and connectors. Repair wiring and connectors as required. Perform TEST VER-2. If wiring and connectors are okay, see INACTIVE FAULT CONDITION . Perform TEST VER-2.
  5. If left O2 sensor was locked on 0.5 volt in step 1), disconnect left O2 sensor connector. Place DRB-II in ohmmeter mode. Using DRB-II, check resistance of O2 sensor connector ground wire (Black/Light Blue wire).
  6. If resistance is more than 5 ohms, repair ground wire for an open. Perform TEST VER-2. If resistance is less than 5 ohms, go to next step.
  7. Connect a jumper wire between left O2 sensor connector signal wire (Black/Dark Green wire) and positive battery terminal. Using DRB-II, read left O2 sensor voltage. If voltage is more than one volt, replace left O2 sensor. Perform TEST VER-2. If voltage is less than one volt, go to next step.
  8. Turn ignition off. Disconnect Powertrain Control Module (PCM) connector. With jumper wire still in place, place DRB-II in voltmeter mode. Using DRB-II, check voltage of PCM connector terminal No. 41, left O2 sensor signal wire (Black/Dark Green wire).
  9. If voltage is more than 10 volts, replace PCM. Perform TEST VER-2. If voltage is less than 10 volts, repair open left O2 sensor signal wire. Perform TEST VER-2.
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