The O2 sensor is located in the exhaust down-pipe, just in front of the catalytic converter. The wire(s) from the O2 sensor are clipped onto the firewall and come up into the engine compartment from below the brake master cylinder, connecting to the car’s wiring harness at the left rear upper corner of the engine compartment. If your car has 3 wires coming out of the O2 sensor, two of them make up the heating circuit for the sensor, and the third wire goes to terminal 24 of the fuel injection ECU. If your car only has one wire coming out of the O2 sensor, it goes to terminal 24 of the injection ECU. In the photos below, the green wire goes toward the ECU, and the black wire leads down to the O2 sensor.   To test the O2 sensor, pull the rubber boot on the black wire back from the connector, exposing the connection:  Attach the positive probe of a digital voltmeter to the connector, and attach the negative probe of the voltmeter to a chassis ground. After allowing a short time for the O2 sensor to come up to operating temperature, the voltmeter should be reading between 1 and 999 millivolts – normally, the readings will fall between 400 and 800 millivolts. Most importantly, the reading should be constantly changing – the O2 sensor sends up to 5 signals per second to the ECU, and if the signal remains constant for more than 2 seconds, the ECU ignores the O2 sensor altogether. If you have no signal, or if the signal is a steady value, the O2 sensor should be replaced. | 
