E9 code, pressure switch remains closed when it should be open, furnace will not start

Safety first: Turn off power and gas before servicing the pressure switch or vent system.

• Step 1 – Inspect tubing for water: Remove the tubing from the pressure switch and inducer. If water drips out, the condensate system is likely clogged. Clear the condensate trap and drain line.
• Step 2 – Check switch at rest: With tubing removed and no call for heat, use a multimeter (ohms) across the pressure switch terminals. The switch should be open (infinite resistance). If it reads closed, the switch is stuck and must be replaced.
• Step 3 – Inspect venting: Ensure there is no constant wind-driven draft or improper vent termination that could keep the switch pulled in.
• Step 4 – Replace pressure switch if needed: Use a Ducane-approved replacement with the same rating (inches water column). A professional should verify correct operation after replacement.