With furnace covers on, inducer starts, pilot starts, inducer runs several minutes, burner (gas) never turns on and furnace shuts off leaving thermostat demanding heat but never starting the furnace again. With covers off the gas valve opens 60 seconds after the inducer and pilot turn on then the blower starts about 60 seconds later (normal operation). Can't get it to fail with covers off. One other item, sometimes but not always if the blower is switched on at the thermostat (not auto) the heat cycle works correctly. There is a "pressure switch" on what looks like a small vent duct off the exhaust duct. In the past I have reset it by pressing a small black plastic stem in the middle between the spade connectors but it just feels mushy now and won't click reset like it did before. What is this switch for and can I buy a replacement since mine is all rusted?
It's an old 80% Day and Night, horizontal install. I believe the Model # is 376BAW04809 and the main control board is #HH84AA009

Sounds like its needing air, is the filter clean,, are the return air vents open and clear,, the switch is supposed to feel that way mushy,when it hasn`t been tripped,

The furnace runs enough to raise the temperature 1 or 2 degrees at the most then stops. If I turnit off then on again, system kiks in and runs again for another 1 degree then stops again. It doesn't hold the pre-set temperature. We go to bed at 70 and we wake up at 62...