American Standard AUH1B080A9H31A Error Code 3 Flashes: Draft Pressure Error

What Does Code 3 Flashes Mean?

A 3-flash code on the Trane White-Rodgers 50A65 board means the vent pressure switch failed to close when the inducer (draft) motor is running, or it opened during operation. The pressure switch is a safety device that confirms the inducer motor is creating enough draft to safely vent combustion gases out of the furnace. Without this confirmation, the control board will not allow ignition.

The inducer motor creates negative pressure inside the heat exchanger, pulling combustion gases through the heat exchanger and out the exhaust vent. The pressure switch measures this negative pressure via a small rubber hose. If the switch does not detect sufficient draft, it stays open, and the board halts the ignition sequence.

The most common homeowner-fixable cause is a blocked exhaust vent or intake pipe. On high-efficiency (90%+) furnaces with PVC vent pipes through a sidewall, ice, snow, bird nests, wasp nests, or debris can block the pipe opening. On conventional furnaces venting through a chimney, similar obstructions can restrict draft. Checking and clearing the vent termination is the first thing to do.

How to Fix It: Check the Exhaust Vent and Intake Pipes

What You'll Need

• Flashlight
• Ladder (if vent terminates above ground level)
• Gloves (for clearing debris)

Steps

1. Turn off power AND shut off gas supply Switch the furnace power OFF. Turn the gas shutoff valve to OFF (perpendicular to the pipe). Wait 5 minutes. If you smell gas at any point, leave the area immediately and call your gas company.
2. Identify your vent type Determine how your Trane furnace vents. High-efficiency models (90%+ AFUE, like XR95 or XV95) use PVC pipes that exit through a sidewall — usually two pipes (exhaust and intake) terminating on an exterior wall. Standard-efficiency models (80% AFUE) use a metal vent pipe going to a chimney. Find where your vent exits the house.
3. Inspect the vent termination outside Go outside and find the vent pipe opening(s). On sidewall-vented furnaces, check both PVC pipes. Look for: ice or snow buildup (common in winter), bird nests, wasp nests, leaves or debris, or anything blocking the opening. On chimney-vented furnaces, look for blockage at the chimney cap from the ground if safely visible.
4. Clear any obstructions Carefully remove any ice, snow, nests, or debris from the vent pipe openings. If ice is the problem, you can pour warm water on it to melt it. Make sure both the exhaust AND intake pipes are clear if your furnace has two PVC pipes. Do NOT insert any tools deep into the vent pipes — you could damage the connections.
5. Check for obvious issues inside Back inside, visually inspect the vent pipe run from the furnace to the wall. Look for any sections that have separated, sagged, or have visible water pooling. On high-efficiency models, check that the condensate drain is not backed up (water pooling in the condensate trap or drain line can restrict the vent).
6. Restore power and gas, then test Turn gas back ON (parallel to pipe). Restore furnace power. Set the thermostat to call for heat. Listen for the inducer motor — it should start immediately when the furnace responds to the heat call. Within 30-60 seconds, you should hear a click (pressure switch closing) followed by the igniter glowing and gas igniting.

When to Call a Professional

Contact a licensed HVAC technician if:

• The vent pipes appear clear but the 3-flash code persists
• You hear the inducer motor running but it sounds weak or different than usual
• There is water pooling inside the furnace or around the condensate drain
• The pressure switch hose (small rubber tube) is visibly cracked or disconnected
• This code appears repeatedly despite clear vent pipes (may indicate a failing inducer)