I have a 2002 Buick Rendezvous and I recently replaced the thermostat. It will blow only cold air, after the car has been on for 30 mins or longer the air might get warm but not hot.. Any suggestions, please?

Check your anti-freeze level. If its low fill it to proper levels in both the radiator and overflow bucket, then look for a leak. If its not low it could be either a bad thermostat or the blend door motor is broken or bad.

I'm having problems too. My car just started overheating. My heat was blowing cold and the temperature went in the red. I put antifreeze in the car and it did the same thing. I looked under the car for a leak and didn't see anything dripping. Any idea what may be wrong?

If you can't find an external leak and you had to add that much anti-freeze sounds like it could be a blown intake or head gasket (s). Pull the oil dipstick while the engine is cold. If its over full and/or foamy there's where the anti- freeze went.

blocked heater core

I have a 2002 buick rendeyvious it was overheating water in radiater was boiling .it was the cooling fan fuse. .i went and bought a 5 dollar fuse and its good again