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Thre are several thing that can keep the switch from closing, they are: a blocked or partially blocked exhaust vent, a weak draft fan, a clogged port where the other end of the tubing connected to the switch is attached to the furnace. You don't give me a model number so I don't know if this is a condensing furnace but if it is a clogged exhaust condensate drain can also keep the switch from closing
Did you check the condensate drain? If it's clear then then you need to check where the vac tubing on the switch connects to the furnace. You can poke a paper clip into the ports (not into the switch). Beyond that you'd need a maometer to check the draft pressure to determne if the fan is doing it's job or not.
You reset by powering the unit down waiting a couple minutes then power it back up.
If gas supply was an issue you'd get a different error (failed ignition). Your unit hasn't made it far enough into the sequence to ignite. You might want to call in a pro instead of dropping all that money on a manometer.
If you're sure the vent/intake is clear then a poor draft reading would indicate something is in the fan itself or you need a new fan.
Just to test try disconneting the exhaust and intake pipes from the furnace and restart it. If the unit advances beyond the pressure switch error (igniter initializes) then you need to dig deeper into the exhaust/intake pipong. You can try a shop vac to suck out the piping.
Ok, disconnect them 1 at a time then both if needed. Start with the intake
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If you're referring to the box the exhaust fan or intake is attached to then yes it can be removed but the gasket behind it and the gasket on the fan housing must be tight when you put it back together to avoid exhaust leaks.I'd appreciate a favorable rating.
If you're sure the exhaust and intake are clear along their entire length then next step would be to replace the pressure switch (assuming you're sure its tubing is open/OK). We would put a manometer on the exhaust but they are pretty pricey so if you don't have one a new pressure switch would be a lot cheaper.