I was wondering if there is a way to bench test CO2 sensors on these cars to install used ones since new ones are either expensive or not available. I can get some from a junk yard, but are they good?

There is a process of using a flame heat ( consumes O2 ) and read the resistance after it gets to operating temp

I was never able to do it myself

Videos on Youtube

Remember you are looking for resistance between 2 pins for a Titania based sensor , the other 2 pins are the heater which is required ( you can test cold )

There is a map to installing sensors as the same connectors can be swapped

I have a 4 pin connector map I made several months ago and looking

Do not twist the pigtail wires on removal / installation as very brittle galvanized coated wire , mine had broken wires sticking out the insulation from previous owner

Pic on old broke computer , you can match it up with the car side connector as the colors are only valid on car side