Like I said, that will help with the easy to diagnose stuff. What about the complex problems, that don't show up on a scanner.

For example, the equipment used to calibrate your air conditioning system. Its more complicated then fill to the manufacturer spec. Newer machines will run the air with a temperature probe, and modify the amount of freon to get an optimal amount.

Depending on your location, municipal landfills and fire stations may also accept waste fluids.