York TM9V080B12MP11 Error Code 3 Red Flashes: Pressure Switch Open
What Does Code 3 Red Flashes Mean?
Once the inducer motor starts, the draft it creates should pull the pressure switch closed within a few seconds, proving to the board that exhaust is actually moving. On this 96% AFUE condensing York furnace, if the switch contacts stay open, the board treats venting as unverified and halts before ignition — no gas is released. The furnace essentially refuses to light because it cannot prove the flue is clear.
The manual lists several causes: a blocked vent pipe, a broken or disconnected pressure-switch hose, disconnected pressure-switch or inducer wiring, a faulty inducer motor, or a faulty pressure switch. On a condensing furnace the sealed PVC venting and the condensate path add their own failure points — a vent termination buried in snow or ice, or a sagging pipe holding condensate, can both reduce draft enough to keep the switch open.
This is the opposite of the 2 Red Flashes code (switch closed when it should be open). It also relates to the 6 Red Flashes code, which is set when the switch closes normally at start but then opens four times during operation; three flashes means the switch never closed at all this cycle.
What You'll Notice
- The inducer motor runs, but the furnace never proceeds to ignition and produces no heat.
- The furnace attempts to start, hesitates for several seconds, then stops.
- The problem often appears after heavy snow, ice, or wind, which can block or backpressure the vent termination.
- It can be intermittent, showing up in certain weather and clearing in others.
Common Causes
How This Is Diagnosed
A technician isolates the cause in the order the manual implies: first confirm the inducer is actually running and building draft, then check the vent and intake terminations and piping for blockage or a condensate-filled sag, then inspect the pressure-switch hose for cracks, kinks, or disconnection, and finally meter the switch itself. A homeowner can safely look at the outdoor vent termination for snow, ice, leaves, or nests, but the internal draft, hose, wiring, and switch checks are technician work on this model, which is why this code is not treated as a do-it-yourself repair.
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 outdoor vent and intake terminations look clear but the code keeps returning.
- The inducer motor is unusually loud, rattles, hums, or does not start at all.
- You can see the pressure-switch hose is cracked, kinked, disconnected, or holding water.
- The code comes and goes with the weather, especially on windy or freezing days.
Frequently Asked Questions
Is there anything I can safely check myself for a 3 red flash code?
Yes — you can look at the outdoor exhaust and intake terminations for snow, ice, leaves, or nests, since a blocked vent is the most common cause. The inducer, hose, wiring, and switch checks should be left to a technician.
Why won't my furnace light with this code?
The furnace will not release gas until the pressure switch confirms the inducer is moving exhaust safely. With the switch stuck open, the board cannot verify venting, so it halts before ignition.
Why does this only happen on cold or windy days?
Snow, ice, or wind at the vent termination can reduce or backpressure the draft enough to keep the pressure switch from closing, which is why the code can appear intermittently in certain weather.
✓ Verified against manufacturer service manual — March 2026