Engine light is emmissions related, oxygensensor, cataltic, egr,. Car can run fine. As far as the wire is concerned, electrical is not always easy to find the problem. If they new what wire was the problem from the get go and charged 400 then thats alot. However to find the problem can take hrs sometimes. I am not a shop, we operated a used car dealership, so we encounter problems like these often.
I had a squirrel eat 2 of my plug wires (he did leave a nut behind in payment), then eat another one when I replaced the original meals. Ended up sprinkling chili powder and red pepper over the wires and the ground under where I parked.

