Hello All,
First time poster...

I have an old gas furnance in my greenhouse that I just re-lit the pilot
light tonight for the first time since last winter. The pilot lights fine
and will stay lit as normal. Next I turn the heat up at the thermostat to
run it. this works ok as well. The gas flames, it heats up and the blower
comes on and runs for about 3-5 min. and then all the flames (both pilot and
heating) just cut off. The Blower continues to run like a normal cooling
phase...

Dont think that it can be the thermocouple can it, or it wouldnt light.
Seems like its some high temperature limit switch or something killing the
gas????? dont know where to start. Any help would be great.