I have a 2003 Buick lesabre custom and I’m having trouble with the car not wanting to crank or start. It is a very sporadic thing and doesn’t seem to have a pattern to it. When it does act up, I go to try and start the car and there’* nothing there. I don’t hear the starter clicking or any noise. The only thing that happens is the dash lights go out. I have replaced the battery and have tested the connections and have used the headlights test to make sure it isn’t the battery. I’m at a loss for what it could be. Any help would be greatly appreciated.

Go through the grounds and clean them all.

I'd guess that one or both of the terminals have some corrosion inside or are not connected tightly.

I would agree but the mechanic has already done all of that. There very clean I’ve checked them myself

Starter motor with a dead spot?
I'd try a new starter motor.

I’ve tried tapping on the starter when it acts up and to no avail. Wouldn’t I be able to get it to start by doing that? And would it be sporadic if it was the starter

Okay, good to know. In order to save time and duplication of efforts, be sure to let us know any other actions that have been taken to diagnose this issue.

I'm concerned that the mechanic that did work on this didn't diagnose the issue . . . in favor of swapping parts. Perhaps I don't have the whole picture?

From your symptoms and diagnostic steps, I suspect the starter solenoid has gone bad. I'd replace the starter (with solenoid).

My apologies for not mentioning it in my original post. I appreciate the feedback. I will try replacing the starter and solenoid

I would verify that you have B+ at the starter first.

Agree. It would be best if this was diagnosed properly first. If that'* not available and you're willing to go by my guess from the other side of the Internet, I'd bet my nickel that replacing the starter would fix it.