oil pressure sensor

So my dad's 2003 Envoy has been over heating for a while, but about 2 weeks ago the oil pressure gauge was going low, so he took it to a mechanic, who changed the oil pressure sensor, but when he did, the oil pressure dropped to 0 and stayed there. So he had him change it back, and it's still at...

Do I need to drain the oil in order to change the oil pressure switch? I'm thinking yes, but I want to make sure.

I have a 2006 GMC Envoy Denali with 55k miles. Recently my oil pressure sensor bottoms out to 0. The stop engine warning comes on. Stopping and restarting the engine resets the gauge for about 5 miles. At stops the gauge will bottom out again. There is no noise or change in performance when the...