Have a 7 year old carrier 2-stage 95% furnace. I have cleaned all lines in this unit, checked ground, cleaned flame rod, getting the right flame sensing voltage all limits are working properly, gas pressure correct.
The issue: It will throw a pressure switch code randomly and of course will never do it in front of me. I have been back to this house twice and found the machine locked out. Turned off power and starts up fine. I have started it dozens of times and every thing is normal on 1st stage and 2nd. Pressure switches close upon inducer spin up per spec and stay that way never drop out. This unit was installed with a single stage stat so it uses the board to determine when 2nd kicks in.
My only idea is that when the inducer slows down for stage 1 after initial startup the switch is opening but it didn't do that in front of me. So, I jumpered the w1-w2 last time I was there and it is running fine now without any issues and no lockouts. Ideas?