Several fixes and diagnostics in here, Seville and Tech Tips.
The level sensor uses a reed switch in the tank bottom, with electrical connector. Inside, there's a magnetic float on a shaft - it moves about 0.6" to trip the reed switch.
If the vehicle owner uses various stop leak and coolant additive products, this can clog the float travel. Tank removal and hot, detergent water flush.
The reed switch or electrical connector may fail. Kicker is, the reed switch is not usually available separately. In that case, $58 for a brand new tank on Amazon.
Google "cadillac forums deville coolant level sensor" or something close.
Proper coolant level is right at the tank seam, cold. Your car has a surge tank with the FULL COLD legend and a flat arrow molded into the black top portion of the tank. Follow the arrow across and down to the level at the seam. "Cold" is after an overnight engine off at ambient temperatures.
EDIT: Welcome to the Funny Farm !!! :highfive: You want to read all the sticky posts on this forum's main threads page as well as in Engines; Northstar.
You can see the float through the translucent lower part of this Seville surge tank .................
Bought my coolant reservoir from rockauto in December, mine was leaking but the check coolant message would not come on until it was indeed low.
Replaced the bad one and topped of the coolant, from then on I had a check coolant message.
Checked the connector and it was ok,
Next I called Dorman tech service and learned that they had a bad batch of coolant tanks that the coolant level sensor was wired in backwards.
Tried to reorder from rockauto and the girl told me that if she sent me a replacement that it would most likely be from the same bad batch, so I just did a return and got another tank from another source and it fixed the problem.