My HVAC setup is tanked propane (under 35F), and heat pump (over 35F). Trane XV90 furnace. About 12 years old.

Today, the heating (its about 20F outside) started blowing cold air. When I switch off the system, wait a bit,, and turn back on...it will heat for about ten minutes, then back to blowing cold air.

Of course I suspect the "flame sensor" but cannot find one behind the panel. The burners are behind a cased metal box at the top of the unit. There are several couplings going into it (the burner box). One I know is the "temp trip limit switch", the other I don''t know. But they both have two wires going to them. Flame sensor is only supposed to have one.

THere are three screws to the burner box, but I have not (yet) removed them because I don't see a "single wire" going in (that would lead me to suggest there is a flame sensor inside).

There are stickers around the system stated it has been "modified" for this particular propane setup, so I don't know if that makes my "system guts" different from most.

I've Amazon ordered an exact "flame sensor" and "temp" limit switch. As stated tho, I have no idea where the flame existing flame sensor is. Or does the temp limit switch work as such?

Any thoughts out there are much appreciated!