My experience with a reader verus a scan tool is the amount of money you want to spend on a shot gun!

Both only point you in the general area of the problem and splatter a bunch of imformaton!

Example. Code stats possibly miss fire on number three cylinder. Bad oxygen sensor in right bank.

Is it a spark plug, a wire or air leak at the intake. Some fords have six sensors in the whole system. One ahead of the air pump and one on each side of the catalytic converter doubled for dual exhaust! Shot gun blast at the area.

Check Engine Light is the shops best coworker for getting work in the shop. If the mechanic works on these day in and day out probably already knows from past experience (paying diagnostic customers)and just changes the part. Charges for it and if it doesn't work he puts your good part in his box for his next trial and error. Change the next most often part and if you are a regular gives you a discount off on the parts.

You can do the same thing just by learning to know how your car systems work.