1997 Deville Coolant Cap

Push down and turn a bit more counter clockwise

It has a safety latch. Idea to first allow pressure to escape without allowing the cap to come off in one turn...to avoid hot coolant all over the place.

You should be able to get it off by taking it to that safety position, then push down slightly and twist counter clockwise.

If it is really jammed, take it to a local radiator shop and ask them to take a peek at it.

Dunno where the sensor is on a 97 Cad. It may be at the top of the radiator or over on the coolant recovery tank, or both may trigger the light. Check the coolant recovery tank, which is usually a plastic bottle connected by hose to the top of the radiator. If that is low, try filling it first with the proper antifreeze mix. I'd also still check the radiator level, or perhaps just have the cooling system flushed and checked since you have no idea how well the prior owner took care of it.