Furnace starts inducer but does not ignite; LED flashes 3 times repeatedly on Armstrong Air control board.

Safety: Turn off power and gas before disassembling venting or inducer components. Use caution on ladders or roofs when inspecting exterior terminations.

Steps to check:

• Inspect vent system: Check the PVC intake and exhaust pipes from the furnace to the outside. Look for sags filled with water, separations, or obstructions at the termination (ice, debris, bird nests). Clear any blockage and ensure proper slope for condensate drainage.
• Check pressure tubing: Remove the tubing from both the inducer port and the pressure switch. Inspect for cracks, splits, or loose fits. Replace brittle or damaged tubing with high-temperature silicone tubing of the same size.
• Verify inducer performance: With the call for heat active, confirm the inducer starts promptly and runs at full speed. Listen for grinding or rattling that may indicate bearing failure. A weak inducer may not create enough negative pressure to close the switch.
• Test pressure switch: With a manometer, verify that the inducer is generating the required negative pressure (inches w.c.) specified on the pressure switch label. If the proper pressure is present but the switch does not close (no continuity), replace the pressure switch.
• Check condensate system: On condensing models, ensure the condensate trap is clean and the drain line is not blocked. A flooded inducer housing can prevent proper pressure sensing.