Oxygen Sensor: Testing and Inspection

Oxygen Sensor Testing:

1. With ignition switch "OFF," disconnect sensor connector.
2. On vehicles with 4 wire connector, connect a high impedance volt/ohm meter between "HEGO SIGNAL" and "HEGO GROUND" at sensor connector.
3. On vehicles with 3 wire connector, connect volt/ohm meter to "HEGO SIGNAL" at sensor connector and battery negative post.
4. Set voltmeter to 20V scale.
5. Start engine and run at idle.
6. Create a vacuum leak to cause a lean condition. An easy way to do this is to remove a vacuum hose from the manifold vacuum tree.
7. Under lean condition, voltage should read 0.4V or less.
8. If not as specified, replace sensor.