We had one installed in our house just over 5 years ago and when we had the really cold spell in December our furnace shut down. Checking the lights showed a slow alternating blink (watchdog mode). After searching on the internet I found out this was likely just a dirty flame sensor. I used needle nose pliers to remove the wire (its on there TIGHT) then removed the 1/4" sheet metal screw that holds it in and wiped it down/scrubbed it with a paper towel. After re-installing it the furnace has been working perfectly for over 1 month now (where before it would only kick on for 30 seconds at a time). When I looked at the sensor it only had a very light white dust on it, if I didn't know better I would have said it looked perfectly fine.
We first noticed the problem when the house had problems keeping close to the set temperature. We would set the thermostat to 20*C but the temperature would vary from 16*C to 20*C and the furnace seemed to be cycling short. It did this for about 2 months until one day it just wouldn't kick on at all (waking up to a 3*C house sucks).
I was over at my dad's place the other day scanning some pictures and I heard his furnace start doing exactly the same thing. I go down and check the thermostat and it was 4*C below the set temperature (and showed calling for heat). I checked his furnace and sure enough, slow alternating blinking...
Both of our furnaces are just over 5 years old and are Lennox mid-efficiency furnaces. I just wanted to warn anybody else with a similar furnace about this issue. Hopefully it will save somebody some big bucks when they don't have to call an emergency furnace/hvac guy because their furnace shut down at night on a weekend during winter.
Cliff's Notes:
If you have a Lennox mid-efficiency furnace and are cold, it's probably your flame sensor and it can be fixed for free.