If it's a dual catalyst system, there are 2 front and 2 rear sensors. The front one's read the O2 levels before the catalysts, the rears are post catalyst.
If you only have 1 catalyst, like on a 4.3L engine, there are 3 pre-catalyst O2s and 1 post.
Testing is pretty much the same for all of them, except that the rear sensors are a little tougher. Basically, there should be 4 wires going to each sensor. The purple wire is the one you really want to concentrate on for this part of the test.
Take your DVOM and set it for millivolts, Then backprobe the purple wire at the O2 sensor harness connector (either on the frame or on the side of the transmission, depending on the sensor. Make sure to connect the DVOM ground to bare metal. You should see a varying voltage between about 60-950 mv and varying widely (engine running). Set your DVOM to average, after a few minutes, you should see an average voltage between 450-550 mv. This only works for pre-catalyst O2 sensors. Also, this is not a complete test, since I don't expect you have a good oscilloscope.
The rear O2 sensor is a little more difficult, since the voltage varies less. Basically, the on-board diagnostics will have to tell you if there is a problem there.
The best way to test all the sensors is to take each one individually, and start with a dead-cold engine. Back-probe the purple wire and monitor the voltage after you turn the key on. As the heater warms the sensor, the voltage will drop. I like to use a stopwatch to time how long it takes to get below 50 mv from the time I turn the key on.
This will be a very time consuming test, since you'll have to do it 4 times, each time waiting for all the sensors to fully cool before testing the next one.
All 4 sensors should be fairly consistent in the amount of time it takes for them to get below 50 mv, and each should take less than 2 minutes to get there.
There are many different tests that you can perform, depending on the symptoms. What kind of problem(s) are yo having? SES lamp on? Sluggish/non-existent acceleration? Black smoke? Failed emissions test?
If you have an SES lamp on, do you know the code? For a failed smog check, do you have the gas readings?
All of these things will play a part in how I would go about diagnosing O2 sensors.
BTW, up until about 4 months ago, I was a GM tech. I am an ASE certified Master Automobile Technician and a licensed California Smog Technician. I probably know more than I should about your truck <img src="/forums/images/graemlins/kewl.gif" alt="" />
