LENNOX control board WATCHGUARD problem

[skydriver]

Hi,

I have 8 year-old LENNOX 80MGF-2, with EGC-1 BURNER/IGNITION/BLOWER CONTROL BOARD. Everytime when I turn on either thermostat panel power or the furnace electrical power, the burner works fine, and room temp. goes up until the setting value(let's say 21 degC), the furnace stops, but diagnostic code shows WATCHGUARD MODE.

The probelm is the next cycle. The room temp. drops down(thermal still displays the setting temp., like 21 deg C, I think there is tolerance), and thermostat is actived to trun on the blower, then ignition trial, but not flame(Burner don't work!), ignition tries three times, and system lockout,blower stops a couple minutes later. The diagnostic code shows the WATCHGUARD, two LED flashing alternately, burners have failed to ignite. The thermostat panel shows HEAT ON.

The funny thing is when the the room temp. keeps dropping down until to 20 deg, which is different from the setting value, the furnace work fine, LEDs fast flashing.After temp. goes back to the setting, burner stops and LED shows WATCHGUARD" again.

I am just wondering this is normal operation of the LENNOX80MGF?

Thanks for any comments.