The fact that the furnace is 6 years old, there's a 100% likelihood that it's a high energy furnace
they still sell 80% furnaces - $500
the "rain" is more than likely condensation which the new furnaces produce.
it sounds like it only happens when it rains
most likely not condensation based on the limited information provided
I'm also thinking that since this person posted this several month ago, the problem has since been resolved and the tech found a malfunctioning CPU.
the motherboard has nothing to do with rain or condensation
if the motherboard failed the furnace would NOT work and there would be NO condensation on a non-running furnace