The first part being if you do an open circuit voltage test with a power probe (not using the rocker switch, just reading voltage) does it load the circuit and is it a viable way of testing a circuit or should you just stay away from open circuit testing all together?
The second one is that I just finished watching one of Eric O's videos (huge fan of Eric O and Paul Danner) and he was testing the signal wire on an upstream 02 sensor and he used a power probe (newer than what I have, with a digital display) to test the signal wire by applying a power and a ground and seeing how the computer responded. Can I do this with a power probe 3 or do the newer ones have a lower amperage driver test function. I like my bacon fried, not my PCMs.
Power Probes can be very useful but use with extreme care it is very easy to overload the circuit by applying a ground you are testing and burn out the engine ECU or a module. When using dummy load to test the power supply to a circuit it has have a resistance in the same ballpark as the working load.
Also be aware the voltage displays on Powerprobes are far from ideal as voltmeters and won't give useful results on some circuits. The shunt resistance of the Powerprobe IV is 350K Ohms that sounds a lot but puts a load on the circuit you are measuring is enough load to pull down the voltage reading on electronic circuits. Measuring voltage with typical digital multi-meter will not pull down the voltage to any degree because the shunt resistance of a digital multimeter is about 10M Ohms. So while a Power Prove IV is fine for checking power to screen wiper, or an ignition coil or trailer wiring but if you use it on an electronic circuit with a high impedance (high resistance) such as a wheel speed sensor you will pull the output down and will not get a useful reading.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
