Customer was informed by a separate contractor that the second stage heat exchanger was cracked and needs to be replaced. - Tech found no cracks or lacerations in either heat exchanger. Found pressure switch hoses backed up with water and drain box was clogged. Found plastic fresh air intake melted to the burner cage. Intake is almost completely obstructed and preventing burners from receiving oxygen. System runs temporarily but overheats in the burner compartment. Checked gas pressures and confirmed proper startup. Need to order replacement intake vent and reconfirm operation. - Tech flushed all pressure switch lines, cleaned flame sensor and igniter, and removed water from draft compartment. - Customer invested in a one system premium service agreement, paid monthly. Left power off to system.