I won't say the cheap code readers won't work in some cases because they do.
The generic code readers have made me so good money. I bought a car for $500 because the generic scanner didn't read all the sensors and they couldn't fix it with the codes it listed. I hooked up my nearly antique SnapOn scanner on it, got the stored codes, located the bad sensor, replaced it and sold the car for $4K. If you have one of the cheap code readers and it helps you fix your car, that is great but if it doesn't find someone with a good scanner to scan it before you buy a bunch of parts that you don't need.
