2000 Impala Heater not working

I have a 2000 3.4 Impala my heater doesn't come on till the car gets warmed up and only if I'm going 65 mph or over, but if I turn the fan on it goes cold. Also fan blower doesn't come on except if it's on high. Even then it blows cold air.

Does the temp gauge read normal?

is the check engine light on?

Is the coolant full?

The fan only working on high is likely a bad blower motor resistor

The coolant is full. The temp gauge reads high sometimes and the low coolant light comes on when it goes high.The check engine light is not on.

Sounds like you have air in the system. Bleed it out and see if that helps.