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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 O2 sensor 1/2 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 .29-.72 volt for 30 seconds, 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, monitor downstream voltage for a few minutes until downstream voltage stabilizes at .4-.6 volt. Downstream oxygen sensor voltage may also be referred to as 1/2 O2 sensor voltage. Once downstream oxygen sensor voltage is .4-.6 volt, use scan tool, actuate oxygen heater test while monitoring downstream oxygen sensor voltage for 2 minutes. Oxygen sensor heater test may also be referred to as O2 sensor heater test. If downstream oxygen sensor voltage remains .4-.6 volt, go to next step. If downstream oxygen sensor voltage does not remain .4-.6 volt, go to step  9.
  2. Turn ignition off. Disconnect connector at downstream oxygen sensor. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter.
  3. Inspect downstream oxygen sensor connector for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, go to next step. If terminals are corroded, pushed out, miswired or damaged, repair as necessary. Perform TEST VER-5A .
  4. Using ohmmeter, check resistance between ground and terminal No. 3 (Black/Light Blue wire) on wiring harness side of connector for downstream oxygen sensor. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit on Black/Light Blue wire between downstream oxygen sensor and PCM. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly. Perform TEST VER-5A .
  5. Turn ignition on with engine off. Connect jumper wire between terminals No. 1 (Tan/White wire) and No. 3 (Black/Light Blue wire) on wiring harness side of connector for downstream oxygen sensor. Using scan tool, read downstream oxygen sensor voltage. If downstream oxygen sensor voltage is .2 volt or more, go to next step. If downstream oxygen sensor voltage is less than .2 volt, replace downstream oxygen sensor. Perform TEST VER-5A .
  6. Turn ignition off. Disconnect connectors at PCM. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly.
  7. Using ohmmeter, check resistance between terminal No. 1 (Tan/White wire) on wiring harness side of connector for downstream oxygen sensor and PCM connector terminal No. 51 (Tan/White wire). If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit on Tan/White wire between PCM and connector for upstream oxygen sensor. Perform TEST VER-5A .
  8. Inspect PCM connectors for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, replace PCM. Perform TEST VER-5A . If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A .
  9. Ensure ignition is off. Disconnect connector at downstream oxygen sensor. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter.
  10. Inspect connector for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, go to next step. If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A .
  11. Ensure ignition is off. Disconnect connectors at PCM. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly.
  12. Inspect PCM connectors for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, replace downstream oxygen sensor. Perform TEST VER-5A . If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A .
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