I've worked on several different systems. Even the most simple systems have many failure points. The more complicated have to communicate with a few other systems. Miscommunication or faults in those systems can cause havoc.
When I was working on anything that came thru the door, I had a subscription to AllData to get troubleshooting and repair info.
I scored a few different mfg factory diag machines that paid for themselves the first time I used them.
There are some great aftermarket machines out there that can do some amazing things, but you must know what you're getting into or you can blow a ton of $$$$$!