I have a White-Rodgers 50A50-473 control board on a Trane XE-80. When trying to heat, the burner does not ignite. The controller indicator flashes twice, indicating a problem with the pressure switch. How do I check whether this is correct and replace the switch?

Maybe this can get you started. Do you have an ohm meter?

How to Fix a Pressure Switch Error on a Trane | eHow

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Connect the ohm meter to the pressure switch. When the system calls for heat and the inducer fan begins to run, check each pressure switch terminal. The ohm meter should read very close to zero. If the ohm meter comes back with an open line reading, the switch is bad and needs replaced. If the pressure switch is functioning properly, other problems may be a plugged vent piping or the inducer fan.

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And this may also be useful,

Pressure switch Trane XE 80 | Archives.HvacMechanic.com

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Disconnect the hose from the pressure switch, blow back through the hose into the other fitting it attaches to. You should be able to blow through it easily. If you can not then the pressure port on the inducer may be plugged.

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Please let me know how it works out.