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DTC P0140: O2 Sensor 1/2 Stays At Center

NOTE: For connector terminal ID, see CONNECTOR IDENTIFICATION . For wiring diagram, see WIRING DIAGRAMS article.
NOTE: The 1/2 O2 sensor is referred to as downstream oxygen sensor in this test. Downstream oxygen sensor operation is monitored when engine is running for more than 2 minutes and engine coolant temperature is greater than 170°F (76.7°C). If downstream oxygen sensor signal voltage is .35-.55 volt for one minute, this causes the Powertrain Control Module (PCM) to pull the O2 sensor voltage to 5 volts. DTC may be stored in PCM if the O2 sensor voltage remains at 5 volts. Possible causes are: defective downstream oxygen sensor, defective PCM, or defective connectors or wiring.
  1. Turn ignition on with engine off. Using scan tool, read Diagnostic Trouble Codes (DTCs). If global good trip counter is displayed and equal to zero, go to next step. If global good trip counter is not displayed or displayed with a value, go to step  5.
  2. Ensure ignition is off. Disconnect downstream oxygen sensor connector. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter. Turn ignition on with engine off. Using scan tool, monitor downstream oxygen sensor voltage. Downstream oxygen sensor voltage may also be referred to as 1/2 O2 sensor voltage. While monitoring downstream oxygen sensor voltage, connect jumper wire between downstream oxygen sensor connector (harness side) terminals No. 4 (Tan/White wire) and No. 3 (Black/Orange wire). If downstream oxygen sensor voltage changes from 5 volts to less than one volt, go to next step. If downstream oxygen sensor voltage does not change as specified, replace downstream oxygen sensor.
  3. Using ohmmeter, check resistance between engine ground and downstream oxygen sensor connector terminal No. 3 (Black/Orange wire). If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit on Black/Orange wire between downstream oxygen sensor and PCM. PCM is located in engine compartment, between air cleaner and radiator. Perform TEST VER-5A .
  4. Disconnect jumper wire. Disconnect PCM connectors. PCM is located in engine compartment, between air cleaner and radiator. Check resistance of Tan/White wire between downstream oxygen sensor connector terminal No. 4 and PCM connector terminal No. 51. If resistance is less than 5 ohms, replace PCM. Perform TEST VER-5A . If resistance is 5 ohms or more, repair open Tan/White wire. Perform TEST VER-5A .
  5. Inspect wiring and connectors between downstream oxygen sensor and PCM. Inspect related wiring harnesses and connectors. Refer to Technical Service Bulletins (TSBs) for related problems. If no problems exist, test is complete. If any problems exist, repair wiring or connectors as necessary. Perform TEST VER-5A .
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