Q: My oil light does not come on when the oil is low, and the heater only blows hot air for a short time.

I have 2 issues, the oil light and heater. My heater all of a sudden has quit working. When I turn on the heater, it will blow out hot air for about 20 minutes and then blow out cold air. Do you know what might be causing this? Another mechanic at a dealership said that if it was the thermostat, that a code would come up. I have recently had the codes read and the only thing coming up in the codes was for the Transmission Control Module, which I have an appointment to be replaced. I had recently asked a question about why my oil light does not come on when the oil is low. I have been so low on oil a couple times that my car has started to make noises before I realized what was wrong. I want to avoid this happening again. I was going to get the oil sensor changed, but the mechanic on here who responded to me said that my oil light does not work that way. How do I know if the oil sensor is bad? And when the oil is low? I cannot afford a diagnostics at this time. Thank you!

My car has 144000 miles.
My car has an automatic transmission.