I have a 99 Malibu and my heater doesn’t seem to be working. The engine temp gauge sits at ¼ and won’t go past that. The heater will blow lukewarm air out, but won’t blow hot air. The thermostat has already been replaced on it, but it looks like that wasn’t the problem . Any ideas?

It sounds like the antifreeze has gummed up the heater core.

Still sounds like the thermostat. Sounds like it is sticking open and it is not letting the coolant sit in the engine long enough for it to warm up. It may just be circulating the coolant from the radiator to the motor constantly.