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Your right, a OBD-II scanner is a must on all new cars.
It can either direct you to what is wrong with the car or at least lead you into the right direction to diagnosing it.
Yes, most people are afraid to work on their modern cars but a engine is a engine is a engine whether it's from a 1 cylinder lawn mower engine to a 10 cylinder.
I was afraid at one time to remove the fuel rail and fuel injectors but I took the fuel rail and fuel injectors off another engine just like mine or close to it and found out how easy it to do.
Most fuel injectors have a lower and upper O ring.
Most fuel injectors are on a fuel rail like pipes or a solid aluminium rail tube.
Most fuel injectors are ok and just need cleaning unless the copper coil inside them burn out.
You can try and get a fuel injector from a junk yard but it's a crapshoot, you either get a good one or one that does not work.
Also when taking the fuel injectors out of the fuel injector ports on the intake manifold make sure you clean the dirt, grime, sand, out before removing, use a micro vacuum attachment that you can hook up to your regular vacuum cleaner.
Yes the professional mechanics might not like what I am saying but most people can learn to fix the most basic things on a car, get a book or manual for your car, a OBD-II scanner.
Most sensors on your car can be replaced yourself and save thousands of dollars.

All that's true, but my comments go back to your original post about aluminum cylinder heads.

With $5 a gallon gas, if you drive 15K miles a year and start out at 18 mpg, there is just no way that swapping out heads is going to save more than $125 a year.

And the possible damage a newbie mechanic is going to do on his car by basically pulling everything off down to the top of the cylinder block, taking the valvetrain apart and reassembling it, and then putting it all together, just isn't worth it for a buck and a quarter.

I am not arguing about the general benefits of working on one's own car, which I totally agree with. But starting out by replacing heads is like starting out at rockclimbing by having a go at K2.