cashcow+

+I have a sob story that may sound familiar, at least to people who have few skills as an auto mechanic. I bought a 99 Ranger in 2000 and it ran with almost no problems till last Feb. the "check engine" light came one, I took it to a mechanic who read a computer code and then swapped out a part and charged me $200 to $300. the next morning the check engine light came one again and I shelled out another 2 or 3 hundred bucks. Two thousand and some dollars later I have a new fuel pump, plugs and wires, mass air flow sensor, new oxygen sensors , etc ect etc, and the morning after the last fix the light came one again. I was under the impression that diagnosis precedes repairs, but apparently that isn't so. It seems that mechanics simply read computer codes and swap out parts. I have become the cash cow of various mechanics in my area, including the Ford dealership, where I bought the Ranger. I would appreciate any advice on what to do, where to go or who to ask.