Mechanic and a Doctor

A mechanic was removing the cylinder heads from the motor of a car when he spotted the famous heart surgeon in his shop, who was standing off to the side, waiting for the service manager to come to take a look at his car.

Mechanic shouted,”Hello Doctor! Please come here for a minute.”

The famous surgeon, a bit surprised, walked over to the mechanic. The mechanic straightened up, wiped his hands and asked argumentatively, “So doctor, look at this. I also open hearts, take valves out, grind‘em, put in new parts, and when I finish this will work as a new one. So how come you get the big money, when you and me is doing basically the same work?”

The doctor leaned over and whispered to the mechanic.,

what doctor would have said ?
(Guess...?)
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(Please try to find out your own answer.)
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Finally Here is the answer.

Doctor said : “Try to do it when the engine is running”.

Moral : Some times small and easy questions have difficult but logical answers. Find the answers out to reach success point.!