Have the noted modulating furnace, IQ Drive stat. When its warmer out, it turns on and runs fine, say above 65F. When it's cooler than that, turn it on and it errors out with "pressure lo open" at the stat. Sometimes from this point it may turn on by itself in about 20 minutes or just sits in that state. Plenty of air supply and exhaust is clear. Both pressure switches replaced. Same symptoms. Maybe one of the new pressure switches defective or error result of different component failure?
check the condensate lines and trap as well as the pressure switch hoses.
water not draining and backing up will cause this problem.
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.