I can easily just copy and paste it from my notes
There are two types of O2 sensors commonly used: Zirconia and Titania. Zirconia can be identified as having flutes on the end. Titania has an exposed flat element on its end. Both types can be bench-tested using a propane torch and multimeter.
1. Set voltmeter on two-volt scale.
2. Connect positive to sensor lead; negative to sensor housing.
3. Hold sensor with pliers, heat with propane torch and record voltage. A good sensor must be able to produce 0.8 volt or higher within 60 seconds.
4. Next, remove sensor from heat source while observing voltmeter. Voltage should drop within 3 seconds.
A sensor contaminated with silicon, lead, burnt oil, or antifreeze will not pass this part of the test.
To test the sensor's heater, connect the leads to an ohmmeter. Any ohm reading is okay as long as it's not an open circuit.
1. Connect black and gray sensor leads to an ohmmeter set to roughly midscale (200K).
2. Hold sensor with pliers, heat with propane torch and observe ohmmeter indicator. After a few seconds, the ohmmeter should indicate an ohmic value. This reading will vary with flame temperature.
3. After observing reading, remove sensor from heat. The ohmmeter should register infinity.
95 E320 Cabriolet, 131K