Lots o' questions on the forums on: - why the truck won't start - why the check engine light is on - whassup with the cooling leak - whyfor doesn't my a/c blow cold Back in 1969 you wouldn't try to fix performance problems without a vacuum gauge, timing light, and tach/dwell meter -- all relatively inexpensive tools available at your corner auto parts place or from the Sears catalog. In that same bygone era you also had to have a cooling system pressure tester and an R-12 charging hose and gauge. Times have changed and the tools have evolved but there is still a basic set of diagnostic tools that you are going to have to have to do any kind of troubleshooting under the hood -- anything more than basic maintenance and replacement of obvious things like belts and hoses. I see the minimum set for today's powertrains to be: 1) Some kind of OBD-II interface. The smart phone ones are the best for the money if you have a smart phone. $50-$100 2) A fuel pressure gauge designed for EFI engines, with a connector that attaches to the fuel rail. $40-$60 3) A cooling system pressure tester with a set of adapters suitable for whatever you're going to work on. $80-$100 4) A refrigerant charging hose with pressure gauge for R-134a. $20 5) A meter (or possibly two separate meters) capable of reading DC volts and DC amps up to 200 amps or so. $100 Now you don't have to run out and buy all this stuff at once but these are such basic tools that if you're trying to diagnose something for more than around 10 minutes and any of these would help then you should run out and get one from your nearest cheap auto parts place or Amazon or whatever. You use them once and they've paid for themselves compared to either a trip to the stealership or guessing and replacing perfectly good parts.