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LOL! If they are good mechanics they should write all of the settings down.
But either way, just for electrical safety.
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Seriously, depending upon the area of the engine you are working on, you can short out things with a wrench, fry components, start the car while its up on jacks, run over yourself.... it does happen.
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Keeps someone from starting the machine and getting you chewed up while you are inside still working on it.
That's the basic idea behind disconnecting the battery whenever you work on the engine. It might seem like it is not needed but it only takes once to get your hand caught in a belt or something worse. Plus, if you are doing something like engine work for a job, your employer will likely insist on it.



