03 century P0128 code
 

Hello All, Recently i acquired my dads 2003 buick century 3.1L with 200,000 miles on it and i checked what codes it would pull since the check engine light is on. Well i think it only pulled the p0128 code according to the print out they gave me but its also suggested to replace everything involving the cooling system even radiator water pump etc. I noticed when i drained the radiator it had a gray looking sludgy gunk stuff around the neck and the radiator cap. Im thinking it had dexcool in it and got mixed with the green coolant but i really dont know so im trying to reach out for advice on where to start and what to check or replace first.

Looks like there could be more than one problem.

If the "stuff" in the cooling system is questionable, it would be a good move to drain the entire cooling system and install new coolant with the correct amount of water to provide freeze and corrosion protection.
BEFORE draining the system, get a new thermostat (195 degree rating) and replace that defective thermostat presently in there , then add the coolant.
Not a difficult job, if you have the means and want to do it yourself. By the way that code causing the C/E lite to come on is due to a thermostat that is most likely stuck open.

Thank you so very much for the input. That is what ill do then.