I took my vehicle to a mechanic who fixed my fuel pump after my car was randomly shutting off while driving. That didn't fix the issue so then he put in a new relay. That didn't fix the issue and now he says he is going to have to install a new computer. He said it 1,000% isn't the spark plugs although I have 108,000 miles on them. How can he be certain they aren't the spark plugs? Is it because of the error code? I just don't want to throw in money when the actual problem isn't being fixed... Also after the engine stops, it doesn't turn back on right away. I have to wait like 30 seconds - 2 minutes for it to start back up otherwise it would just be cranking (I have a 2006 Mustang GT)

 

Sounds like your "mechanic" is just throwing parts at it. Wrong way to diagnose and expensive for you. Did he even duplicate the problem to determine it was the pump? How is he certain it's not the plugs? Based on his previous hunches, he was clearly wrong.

That being said, I would agree that plugs are not the issue. If they were, poor gas mileage, misfires, and hard to start when cold would be very obvious.

I think you need to either question him about how he's diagnosing things or find another shop. If he gets defensive about you asking what's making him think this-or-that, run (don't walk) to another shop, he clearly has no clue what he's doing or, rather than diagnose correctly, having you pay for his laziness in (actually) diagnosing the problem.