My elderly neighbor has a Coleman wall furnace that appears original to the home built in 1957. The pilot would not stay lit so I replaced the thermocouple and everything was fine for about a week. Now the pilot will stay lit for a day or two and then self-extinguish.

This is an old unit and does not have a thermostat but operates manually much like a gas range with a ball valve that regulates gas flow. I see no source of electricity to the furnace.

Is the pilot good and strong?

Pilot flame appears to be good enough. When working, the burner will light very quickly. The valve controlling propane to the pilot is fully open. The thermocouple sits off to the side of the wide pilot flame but this is the way it was designed and worked for 50 years.

I would assume that once the t/c reaches operating temperature it should be good to go. There are no drafts or anything I can see that would disturb the flame causing it to go out.

The thermocouple actually generates voltage called millivoltage. This curent keeps the safety of the valve opened. The thermocouple should be in the pilot flame about 3/4 inch or more.

Thanks for your help. I repositioned/modified the t/c to get it further into the flame and for now it seems to be working.

Also, thanks for the info regarding the minor electrical voltage. I saw some wires extending from the coupling at the end of the t/c and ending at a valve but saw no source of power coming in. I wondered what was going on.

You could have a millivoltage generator instead of a thermocouple. As long as it is working that is what counts.