OK, I'll type out the whole thing, which I should have done to start.
Furnace was running for shorter than normal periods, and house wasn't warming up, so I figured it was "short cycling". Noticed at the same time that the wireless thermostat was occasionally showing "no signal". none of this felt urgent, because the house was sort of close to the correct temperature most of the day, so the furnace must have been working most of the time.
LED on furnace board wasn't showing an error, but I followed the procedure in the manual to retrieve stored codes, and it did show a code for "flue limit switch error". Ok, that seems like a common airflow-related issue. Replaced the air filter, checked the combustion air intake, and that's nice and clear. I cannot check the furnace exhaust, as it's on my roof and the roof is covered in ice and snow in that area. I did get on a ladder to look at it, and there's no external blockage anyway. Couldn't be too badly blocked, since it's running fine most of the time.
So I'm sitting here, thinking that I have two things indicating issues, and I don't know which to troubleshoot. The wireless thermostat (Honeywell THM532R sending to Honeywell TH6320R) seems to be losing connectivity sometimes. AND the flue limit switch is showing an error occasionally. Could one problem cause the other? I don't see how, but .....
I have called Atco, and they plan to come out sometime next week.
What am I missing here?