My gas heater is in the attic. When the attic get real cold (near or below freezing) the heater shut down. With a cold attic the igniter will come on, the gas will come on, then the flame, then 20 seconds later the gas shuts off. When the attic is not so cold every thing works fine. Temputure senative. Any suggestions?

It could be the flame sensor being a bit dirty and along with the cold having a effect on the operation of the unit. Other than that testing with a can of cold spray on the components might be necessary to see which one fails to work in the cold weather.

You can try to clean the flame sensor if so equipped on your no model number furnace. Fine sand paper or steel wool will work.