May thermostat staying open. the heater door works on vacuum also, I don't remember the exact symptoms. Not sure if there is a leak and not operating the door correctly, if it would not switch to defrost or not. At least it is warm out today. By sunday you may need some heat though. Thermostat might be a good thing to change anyways if it is original and shouldn't be hard to change, as long as the bolts aren't rusted in.