clogged drain hose. If the drain hose is clogged it might cause the pressure switch to fail. My suggestion: 1. When the heater starts up, give the pressure switch a firm whack. Seriously. That might unstick it. 2. If that doesn't work, see if the drain pipe (that pipe often goes to a floor drain) is clogged with water. Turn the furnace off, pull out the hose that goes from the pressure switch to the drain pipe, and see if water comes out of the hose. If you can get the water drained, the pressure switch may start to work.

 Here's what the pressure switch looks like: