Unit is only 5 years old and no issues when shut off last May. Not to likely the board is gone already. What is the warranty on the board?

Not an expert by any means but I am looking up what is wrong with my heater... It is a trane... I see a lot of post that have the blink three time problem. From what I have read it is your pressure switch. Either it is bad, the vac line has something clogged in it or the vent itself is clogged with a birs nest or squeril or?? what ever... You can test the pressure switch very easy... blow or suck slightlyt on the tube side of the pressure switch and (on mine there are two terminals) connect your volt meter and chech for continuity if no continuity the switch is bad. I am betting something is in your vent with only being 5 years old