I have a Trane xe 80 heater. Two of the flame burners light up but not the other two. When turning on the heater, two will start up, then shut down and all we get is cold air. Only way all flames will burn is by putting the thermostat up to 90 degrees. We have been having to do this in this cold weather - keep it on an extremely high temperature, turn it off, then when the house gets cold again, do this process again. What is wrong with the heater?

Low gas pressure, or burners and/or manifold gas orifices clogged/dirty.

What does the gas valve look like - how can that be cleaned ?- also the manifold how can that be cleaned?

Either/all issue(s) are really a job for a pro, or a really good handyperson.