When the heater only works on high, the most common problem is a blower motor resistor. The correct repair is to replace it with a new resistor.Google+
Car Problem Reports
Buick Century Heater only works on high
Buick Century Problem
Average Mileage: 117,333 mi (73,000 mi - 200,000 mi)
Visitor, 2002 Buick Century, 116,000 mi
My heater AC motor does not work on the lowest two settings.
Visitor, 1999 Buick Century, 73,000 mi
low does not work
My heater only worked on high before, and when I would turn it on it would make a horrible screeching sound. The thermostat turned out to be bad, once this was replaced, it seems to work a lot better.
Visitor, 1999 Buick Century, 200,000 mi
No heat unless I turn it to high
Visitor, 1999 Buick Century, 84,000 mi
heater only works on high
my heater only works on the high setting, when i turn it on high it makes a loud noise and does not blow heat.