2001 Buick Century overheating

Try bleeding air from system like this remove cap from overflow tank fill with antifreeze to full mark leave cap off and start vehicle and let idle you will notice level in tank start to rise this is normal once thermostat starts to open level in tank will drop add more antifreeze to full mark then put cap back on.

make sure no air getting in there of in there turn ac on see if fan goes on if it does that works so not the fan so turn ac off and watch as it goes over half way to hot thr fan should kick on and it should cool down if it doesnt come on then it's a thermostat if it on and has coolant check to see if its brown and milky if it is thats a head gasket if not check the oil to see if coolants in there