York TM9T100C16MP11 Error Code 3 Red Flashes: Pressure Switch Open

What Does Code 3 Red Flashes Mean?

The pressure switch on the York TM9T100C16MP11 is connected to the inducer housing by a small hose. When the inducer runs, it creates negative pressure that should pull the switch closed, proving the flue is clear and exhaust is moving. A 3 red flash code means the contacts stayed open when they should have closed, so the board halts before ignition.

As a 96% AFUE condensing furnace, the TM9T typically vents through PVC pipe, and the most common cause of this code is a blocked vent or intake termination — snow, ice, leaves, or a nest can choke it. Because the design often uses separate exhaust and intake pipes, both terminations should be checked. Other causes include a cracked or disconnected pressure-switch hose, water in the hose, a weak inducer, disconnected wiring, or a faulty switch.

This code is distinct from its neighbors: 2 red flashes means the switch is closed when it should be open at startup, and 6 red flashes means the switch closed normally but reopened repeatedly during a running cycle.

What You'll Notice

• The inducer runs but the burner never lights, and the board shows three red flashes.
• There is no heat despite repeated ignition attempts.
• The vent or intake termination outside may be visibly blocked by snow, ice, or debris.
• You may hear a whistling or gurgling near the inducer if the hose has an issue.
• The code may appear intermittently, especially on very cold or windy days.

Common Causes

How This Is Diagnosed

A technician works from the outside in: the vent and intake terminations are checked for blockage, then the pressure-switch hose is inspected for cracks, disconnection, or trapped condensate. The inducer's draft is measured at the switch, the switch itself is tested for correct actuation, and the wiring is checked for continuity. Clearing an accessible outside termination of snow or debris is a reasonable homeowner check, but the hose, inducer, and switch work belong to a professional.

When to Call a Professional

This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:

• The inducer runs but the burner never ignites and the board shows three red flashes
• The vent or intake pipe outside is blocked in a way you cannot safely clear
• The code returns after the vent termination has been cleared
• The pressure-switch hose looks cracked or disconnected, or has water in it
• The inducer motor sounds loud, rattles, or does not start

✓ Verified against manufacturer service manual — March 2026