York TM9T100C16MP11 Error Code 6 Red Flashes: Pressure Switch Opened During Operation
What Does Code 6 Red Flashes Mean?
A 6 red flashes code on the York TM9T100C16MP11 means that while the furnace was running, the pressure switch opened four times during a single call for heat, triggering a one-hour lockout. The furnace lit and was operating, but it kept losing proven venting draft partway through the cycle.
Because the failure happens mid-run and intermittently, the causes are usually marginal rather than total: a partially blocked vent that flows enough to start but not enough to sustain draft (especially at high fire), a sagging vent section holding condensate, a weakening inducer, or a pressure switch with marginal contacts. As a condensing furnace, the TM9T can also be affected by water pooling in the vent piping.
This is different from the other pressure-switch codes. The 3 red flashes code means the switch never closed after the inducer started, and the 2 red flashes code means it was closed when it should have been open at startup. The 6 red flashes code specifically means it closed normally and then reopened under running conditions.
What You'll Notice
- The furnace lights but shuts down partway through the cycle, then retries.
- After several mid-cycle failures it locks out for about an hour.
- The board shows six red flashes.
- The inducer may sound weak or fluctuate in tone.
- You may notice water pooling in or dripping from the vent piping.
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 checks the vent and intake for a partial blockage or trapped condensate, inspects the pressure-switch hose, and then measures the inducer's performance and the switch's actuation while the furnace runs — since the fault only shows up under operating conditions. The switch is tested and replaced if its contacts are marginal. On windy sites, the vent termination may need a wind guard.
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 furnace starts heating but shuts off partway through and eventually locks out with six red flashes
- You notice water pooling in or dripping from the vent piping
- The inducer motor sounds weak, loud, or inconsistent
- The problem only occurs on windy days
- The code returns after each one-hour lockout
Frequently Asked Questions
What is the difference between 6 red flashes and 3 red flashes?
Three red flashes means the pressure switch never closes after the inducer starts, while six red flashes means it closes normally but then reopens repeatedly during the running cycle.
Why does the furnace lock out for an hour?
The one-hour lockout is a safety response after four mid-cycle pressure-switch failures. The furnace will automatically retry when the hour ends, but the underlying venting issue should be investigated.
Can I fix 6 red flashes myself?
You can check an accessible outside vent termination for partial blockage, but hose, inducer, and switch diagnosis under running conditions is a technician's job, which is why this code is not a do-it-yourself repair.
Sources
✓ Verified against manufacturer service manual — March 2026