Here are some images showing the test in action. They are captured from an 89 Chevrolet pickup 7.4L 109k Miles failing smog tests.
Here is the 2500rpm signal, shows no switching at all. Engine forced full rich produced the same display.
This is the signal with the engine full lean. Ignition spike took reading to 68mv, looks like true minimum was 160mv.
Here's the snap throttle at full lean limit, its fast, but it can't get past 800mv where it should.
After installing the new sensor the 2500rpm reading is now showing some switching. The waveform is not what I'd call perfect, but its much better than what we had. We now have some fuel control back.
Snap throttle with the new sensor shows a good waveform, this is what you should see. Good minimum, maximum volts, and good rise time. Snap throttle on this application is easy, since it stays in open loop at idle. The maximum is reading high, from an ignition spike, true reading is about 890mv.
This truck had its original sensor. The outer metal shield of the sensor probe was completely eroded away. The smog readings at 2500rpm before the sensor change: co 1.25 hc 180 o2 .80 co2 13.5 After the new sensor: co .15 hc 115 o2 1.95 co2 14.1
After the sensor replacement, some small air leaks were found and corrected, the final figures for emissions were: co .09 hc 75 O2 1.10 cO2 14.9