Water temperature not correct

Scan it for codes and come back with that. That being said there are a few immediate checks you can do.

First thing I would do is just to be safe, I would do a head gasket test to make sure the coolant isn't leaking - it's usually not the gasket, but once these old GMs get up there in age, a lot of them need head gaskets and if you are having temp issues, it's worth testing. Get a kit like this: https://www.amazon.com/RELD-Gasket-Teste...

The next thing I would do is replace the thermostat -- the part number I found is GM 12622410 and it can be found here: https://www.gmpartsgiant.com/parts/gm-th... -- don't use a aftermarket one, even a good one like a Doorman part. Too sensitive. This code means the thermostat is bad: P0128

The other thing I sometimes know occurs on these "Old GM" vehicles is oftentimes the cluster fails, but the engine portion is fine -- sometimes it can be rebuilt, but often you need to replace the cluster -- you will need a scan tool which can program the mileage once you replace it so it matches so you do not have a "not actual" mark on your title, but if your state doesn't check after 10 years you will want to write the mileage it was replaced at and if you ever sell the car, you need to mention that the cluster is replaced. I have looked into it and it seems like these pull from the ECU, so you may not need to program it, but keep that in mind. GM part number is 16256936.

To provide context why I said "Old GM", especially if you are too young to remember the 2007/2008 GFC: In 2008, the Obama administration gave GM a stimulus, but made them clean up their books, stop wasting money and forced them to hit agressive emissions goals -- that's why anything after 2008+ is not as bulletptoof as the old GMT800/GMT400 and the passenger cars pre-2008. New GM is not the same as Old GM which was famous for 500k mile DuraMax GMT800 with Allison transmissions diesels which run like they're still new, and the new AFM/turbo engines are a far cry from what they used to be -- they used to have engines like the pushrod 5.7/6.0 Vortec (which was LS baed) and DuraMax diesels like the 2007 LBZ, and the diesels were pre-emissions before 2007.5 -- but the 2007.5s retained the Allison transmissions when they retired the LBZ DuraMax.

Even their newer trucks in the eras of emissions goals went to i4 engines in Silverados which are known to go bad at 115-120k miles with towing. They're not all bad news, but not the same as the 2007 and older platforms.