Ruud achiever 90 plus wouldn't start

Thanks for the carefull description of the fault, The vent system is designed so that if a leak occurs fresh air from the living space will be pulled into the vent rather than dangerous combustion gases pushed out to insure this there is a carefully calibrated -pressure switch that will not allow the unit to fire unless a negative pressure on the vent is proven this can be prevented by many things, here are a a few :a sticky micro switch, a leaking hose from the vent to the pressure sensor, cold stacking in the flu(either a down draft or extremely cold air in vent that restricts warm combustion air from being pushed out), a problem with the power exhaust fan(debris has gotten in the housing preventing an easy start), i usually find the cause to either be a bad pressure sensor hose or debris in ths exhaust fan housing either blocking blade movement or obstructing the pressure sensor hose port.
To clear the fault turn of the breaker to the unit then restart the unit and fallow the sequence of operation to see where the fault occurs the fan should come on first then the exhaust blower should come on when negative pressure is achieved it start the lighting sequence if the lighting sequence doesn't start and you get the same fault take the neg pressure hose loose from the exhaust fan housing and start the sequence again while pulling a slight negative pressure on the pressure sensor with a hand held pump continue this process until you find the fault.