How to Measure the Voltage of an O2 Sensor on an Audi

1. Open the hood of your Audi to locate the oxygen sensors. Two will be before and after the catalytic converter, best accessed from the underside of the vehicle. The other will be on the exhaust manifold. The oxygen sensor looks like a small cylinder with one wire coming out of it. For the exact locations of the sensors, consult your owner's manual.
2. Connect the positive (red) terminal of the multimeter to the wire coming from the oxygen sensor.
3. Attach the negative (black) terminal of the multimeter to a clean portion of your car's chassis.
4. Set the multimeter to measure direct current voltage and enable the bar graph.
5. Turn on your vehicle but do not crank the engine. The multimeter should register a voltage.
6. Read the multimeter's bar graph. A well-functioning oxygen sensor will read about 0.45-volt.