We had no heat, so we then checked the light indicator on the unit and discovered it was a Pressure switch error. Thinking there was blockage in the intake or outflow PVC pipe, we first started with the vent on the outside of the house. We unscrewed the vent cover and put a wooden pole, like a broom handle, into each pipe and there appeared to be no blockage. We then tested the unit with the metal cover off the area where the burners are and the unit worked, leading us to believe there was a blockage SOMEWHERE, maybe close to the area at the top of the unit where the PVC pipes were connected to the unit. This involved cutting the intake PVC pipe with a hacksaw so we could check for blockage. Sure enough, an old wasp nest and several leaves were at the base of the pipe, right above the burner area. Vacuumed out the nest and leaves. We bought a a PVC coupling to reconnect the area we split with the hack saw. We put 1/8" hardware cloth over the vent area outside to deter further wasp nests and are back in the heating business.