On my Rheem XE 80 downdraft gas furnace, I'm getting intermittent heat. Getting error code of 4 blinks on the single red LED, which means "high temp limit open".

The purge fan has been cleaned (it was very dirty) and the vacuum line to the vacuum switch is not plugged. Vacuum switch itselfs works perfectly - tested with a multi-meter. The two flame sensors work fine - have zero resistance. Installed a new igniter - the old one was 14 years old, but the problem persists. I pulled the high limit switch, which sticks 2-3" into the combustion chamber, and it was nice and clean. It shows zero resistance at room temp. Air filters are freshly cleaned.

I installed a new 'generic' White Rodgers board in this furnace two years ago, when the mechanical relays on the OEM board failed. So this board is only two years old.

I don't THINK I have a true high limit problem. The repairman can't come for at least three days. What should I look at next?

Update: seems that there was a broken solder joint on the control board that was causing the "high limit" fault code. The board is only two years old, but was installed with a little bit of sideways stress on wiring to the main molex connector. I'll try to re-solder the post, if that doesn't work, have to get a new board.