Furnace starts inducer but pressure switch does not close; LED on control board flashes twice repeatedly; burners do not light.

Safety first: Turn off power to the furnace before removing any covers. Do not operate the furnace with venting disconnected.

• Inspect vent and intake: For Fraser‑Johnston condensing furnaces with PVC venting, go outside and check that the exhaust and intake terminations are not blocked by leaves, snow, nests, or debris. Clear any obstruction.
• Check condensate drain: A clogged condensate trap or drain line can cause water to back up into the inducer and pressure switch tubing. Inspect the condensate trap and hoses; flush with warm water and ensure they are sloped to drain.
• Inspect pressure switch tubing: Remove the front panel and locate the round pressure switch mounted near the inducer. Check the rubber tubing for cracks, kinks, or water. Replace damaged tubing and blow through the line to clear any blockage (with power off).
• Test pressure switch operation: With a multimeter and power off, disconnect the wires and check continuity across the pressure switch terminals while the inducer is running (requires advanced skill). If the switch never closes even with clear venting and a running inducer, the switch may be faulty.
• Check inducer motor: Listen for abnormal noise or slow start. If the inducer does not start or is very weak, it may not create enough draft to close the pressure switch and should be replaced by a technician.