Should I check the fuses with the key off? Should I check the fuses with the key
on for the power fuses? I have a 2000 deville base.
For what reason are you thinking you need to check fuses? What method are you going to use to check them?
I'm thinking there is a power drain somewhere. I thought maybe there could be a bad fuse. The service engine light also comes on.
A bad fuse will not cause a power drain. You can pull one fuse at a time until the power drain stops then you will know which circuit is causing the problem. You will need an voltmeter / ammeter connected to the battery to measure the current draw. Then remove and reinstall one fuse at a time until the current draw disappears.