Test NTC-7: Checking For Oxygen Sensor Switching

NOTE: For connector terminal ID, see CONNECTOR IDENTIFICATION . For wiring diagram, see WIRING DIAGRAMS article.

NOTE: Upstream oxygen sensor may also be referred to as 1/1 O2 sensor. Downstream oxygen sensor may also be referred to as 1/2 O2 sensor.

Start engine and warm engine to normal operating temperature. With engine running, use scan tool to read oxygen sensor voltage for each oxygen sensor for at least 10 seconds. Downstream oxygen sensor voltage does not switch like the upstream oxygen sensor voltage, as downstream oxygen sensor voltage switches slower than upstream oxygen sensor voltage. If oxygen sensor voltage for each oxygen sensor is switching normally, test is complete. If oxygen sensor voltage for each oxygen sensor is not switching normally, go to next step.
If upstream oxygen sensor remains locked on lean, go to next step. If upstream oxygen sensor does not remain locked on lean, perform TEST NTC-4: CHECKING ENGINE MECHANICAL SYSTEMS .
Shut engine off. Turn ignition on with engine off. Using scan tool, read oxygen sensor voltage for each oxygen sensor. If oxygen sensor voltage is less than .1 volt for upstream or downstream oxygen sensor, go to next step. If oxygen sensor voltage is .1 volt or more for upstream or downstream oxygen sensor, go to step 7 .
Turn ignition off. Disconnect connector at appropriate oxygen sensor in which oxygen sensor voltage was less than .1 volt. Upstream oxygen sensor is located on exhaust pipe, just above the catalytic converter. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter.
Ensure ignition is off. Disconnect PCM connectors. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly.
Resistance should be checked on circuit for oxygen sensor in which the voltage was less than .1 volt. Using ohmmeter, check resistance between ground and terminal No. 4 (Black/Dark Green wire) for upstream oxygen sensor or terminal No. 1 (Tan/White wire) for downstream oxygen sensor on wiring harness side of oxygen sensor connector. If resistance is 5 ohms or more, replace appropriate oxygen sensor. Perform TEST VER-5A . If resistance is less than 5 ohms, repair short to ground on Black/Dark Green wire for upstream oxygen sensor or Tan/White wire for downstream oxygen sensor between PCM and oxygen sensor. Perform TEST VER-5A .
Start engine and allow engine to idle. Check engine vacuum for abnormal vacuum reading. If vacuum is normal, go to next step. If abnormal vacuum reading exists, repair as necessary. Perform TEST VER-5A .
Inspect air cleaner and inlet ducts for restrictions. If no restrictions exist, perform TEST NTC-4: CHECKING ENGINE MECHANICAL SYSTEMS . If air cleaner or inlet ducts are restricted, repair as necessary. Perform TEST VER-5A .