Oxygen Sensor Verification
To verify the Oxygen sensor, make sure the sensor is at operating temp (run above 2000 rpm for two minutes with engine at operating temp, for heated sensor's, make sure the element is good and getting power and ground) and hold engine rpm at 2500 rpm. Force the engine rich with propane and look for maximum voltage reading (should rise above 800mv). After it flat-lines rich, quickly shut off propane and it should reach its minimum voltage reading (should go below at least 150mv). The waveform below gives you an example of what you should see. When the propane is shut off, the sensor signal should drop from its full rich to full lean limit.
Before the computer gets a chance to take over again, snap the throttle wide open while its still full lean and release to idle position, and see how fast it rises to its full rich reading. If it gets there in less than 100ms, the sensor is adequate. If you want precise control look for faster than that. The image below illustrates how the signal will look. If it can't get to less than 150mv, or more than 800mv, or if the lean too rich speed is more than 100ms, the sensor is due for replacement.
I suppose there's a fine line here, if the numbers are close, and its switching good, I suppose you could still call it adequate and fuel control would also be adequate. But if you are dealing with emissions testing, you will need to get the fuel control as tight as possible. If the sensor can't tell the computer the exact quantity of what is flowing past it, or takes forever doing it, the computer can't control fuel fast enough to permit the Catalytic converter to reduce emissions efficiently.
Go to this page to see the test in action.
This test can be performed on the sensor installed at the outlet of the Catalytic converter to verify its condition as well. These are looking at Oxygen levels on the outlet side of the Cat, and should remain flatlined if the Cat is working efficiently. Since it remains flat, do the snap throttle after a two minute run at 2500 rpm and look at the numbers. Here's a sample.
When you start verifying Oxygen sensors, you will save a lot of time not replacing them in hopes of correcting some driveability problem you can't seem to nail down. Once you know the sensor is good, you can forget about it being the cause of the problem you are trying to correct.