LED flashes code 31; furnace will not ignite, inducer may run but pressure switch does not prove.

Safety: Turn off power and gas. Do not operate the furnace with a known venting problem; carbon monoxide can accumulate.

Steps:

• Inspect vent pipe: From the furnace to the termination, check PVC or metal vent for sagging, water accumulation, bird nests, leaves, or ice blockage. Clear any obstructions.
• Check condensate drain (condensing models): Locate the condensate trap and hoses. Remove and flush with warm water to clear slime or debris. Ensure hoses slope downward with no kinks.
• Inspect pressure switch tubing: Remove and clear the rubber tubing between the pressure switch and inducer/collector box. Check for water or cracks.
• Test pressure switch: With a multimeter, verify that the switch closes (shows continuity) only when the inducer is running. If it never closes despite good draft, the switch may be faulty.
• Observe inducer: Ensure the inducer motor starts promptly and runs smoothly without excessive noise or vibration.

When to call a technician: If venting appears clear but code 31 persists, or if the inducer is weak or noisy, a technician should measure draft pressure and replace the pressure switch or inducer as needed.