2002 Mazda Tribute vehicle brain/ control system

What is the symptom? Diagnostic fault codes are stored in the Engine Computer, so if you unplug it to replace it, that loss of power erases the codes in it. Any replacement computer won't have any codes in it until you turn on the ignition switch and a problem is detected.

There isn't much you can do to damage an Engine Computer. To go through three of them suggests there's some other cause for the symptoms. What exactly is happening now?

When you have a coolant light on the dash, that's for the level in the reservoir. That doesn't have anything to do with the systems monitored by the Engine Computer so there is no fault code related to that.

I'm not sure what your mechanic cleaned, but I've taken a lot of computers apart to install defects that I can switch in and out for my students to practice troubleshooting. The circuit boards are sealed really well because of the environment they live in. They can take some rough handling without any problem unless there is already a corroded spot that is about to act up.

There's a couple of things you might consider. First of all, there is a possibility your replacement Engine Computer will not work until it has your vehicle ID number programmed into it. General Motors is one of the world's leading manufacturers when it comes to innovations that benefit them and cost their customers lots of money. To my knowledge they were the first manufacturer to pull that stunt starting with some 2002 models. While I hadn't heard of that with any other manufacturers that far back, it is possible that applies to your vehicle.

The next thing that comes to mind is if your coolant light came on due to low level in the reservoir, where did it go? Unless you see evidence of a leak, the most likely suspect is a leaking cylinder head gasket. Depending on where the leak is and how bad it is, that can cause poor performance, overheating, a misfire, and / or white smoke from the tail pipe. There's two good tests for that. One is to add a small bottle dark purple dye to the coolant, then search a day or two later with a black light. The dye will show up as a bright yellow stain. If the head gasket is leaking, you'll find that stain inside the tail pipe. The second test involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.