York TM9V080B12MP11 Error Code 6 Red Flashes: Pressure Switch Opened During Operation
What Does Code 6 Red Flashes Mean?
Unlike the 3 Red Flashes code, where the pressure switch never closes, this code is set when the switch does close and lets the furnace run, but then drops open four separate times during the same call for heat. After the fourth interruption the furnace enters a one-hour lockout and then retries automatically. It describes an intermittent loss of proven venting rather than a total failure to start.
The manual points to a faulty inducer, a blocked vent pipe, or a faulty pressure switch. In practice a partial restriction is the classic trigger: enough draft exists to close the switch at startup, but not enough to hold it closed steadily — especially when this two-stage furnace steps up to high fire and demands more airflow, or when wind gusts backpressure the flue. On a condensing furnace, condensate pooling in a sagging vent section can create the same on-again, off-again restriction.
Because the failure is intermittent and tied to draft under load, it can be harder to catch than a switch that is simply stuck, and it often shows up more on cold or windy days when the furnace runs hard.
What You'll Notice
- The furnace lights and runs, then shuts down partway through the cycle, sometimes repeatedly.
- After several interruptions the furnace goes quiet for about an hour, then tries again.
- The problem is worse on windy days or when the furnace runs at high fire on cold days.
- You may hear the inducer laboring or sounding inconsistent during a cycle.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Blocked vent pipe | Most common | ✗ Call a pro → |
| Faulty inducer motor | Common | ✗ Call a pro → |
| Faulty pressure switch | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician looks for a restriction that only bites under load: checking the vent and intake for partial blockage or a condensate-holding sag, verifying inducer performance and draft when the furnace steps to high fire, inspecting the pressure-switch hose, and metering the switch for marginal contacts. A homeowner can safely check the outdoor vent termination for snow, ice, or debris, but confirming inducer draft and testing the switch under operating conditions is technician work, 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 look clear but the furnace keeps dropping out and locking for an hour.
- The inducer sounds weak, loud, or inconsistent during a cycle.
- The problem appears mainly on windy days or when the furnace runs at high fire.
- You find water pooled in the vent piping or a cracked pressure-switch hose.
Frequently Asked Questions
What is the difference between 3 red flashes and 6 red flashes?
With 3 red flashes the pressure switch never closes, so the furnace won't ignite at all. With 6 red flashes the switch closes and the furnace runs, but then the switch opens four times during the cycle, causing a one-hour lockout.
Why does my furnace run for a while and then quit with this code?
A partial vent restriction or a weak inducer can provide enough draft to start but not enough to hold the pressure switch closed under load, so the switch opens mid-cycle. After four such drops the furnace locks out for an hour.
Will the furnace fix itself after the one-hour lockout?
It will retry automatically after an hour, but if the underlying restriction or inducer problem remains, the code will return. It should be diagnosed rather than left to keep cycling.
✓ Verified against manufacturer service manual — March 2026