Took Buick to local Chevrolet dealer in Baytown TX. They replaced thermostat and indicated the heater core was defective. Quoted me $1,390.00 to replace heater core, and I refused work. Charged $400 for technical trouble shooting work. Took Buick to a local shop in my neighborhood, and they replaced the heater core with lifetime warranty, new radiator with lifetime warranty, new water pump with lifetime warranty, new antifreeze coolant, and both radiator hoses for $1,205.00 total cost. No joke. Drove car home, and before I could drive 3 miles, the dash HOT engine light came on again. It must be an electrical issue. Has anyone experienced this issue, and how can I repair? Thank you.

Update from Dec 29, 2016: After checking "Engine running hot issue" on internet, I was told that replacing the intake gasket might correct the problem. Took car back to repair shop and had the intake gasket replaced. No improvement.

Did engine pressure compression test, and found system leaking pressure. They concluded the problem was either a head gasket, a cracked/warped head, or a cracked block.

I did not want to fight this issue anymore, so I traded the Buick in on a new car. They gave me basically nothing for the trade in, so I do not feel badly about the problem. $2,400 trade in is less that I spent on the cooling repairs.