York TM9T100C16MP11 Error Code Pressure Switch Opened During Operation: Pressure Switch Opened During Operation

What Does Code Pressure Switch Opened During Operation Mean?

The 6-red-flash code on the York S1-33103010000 board is a soft lockout triggered when the pressure switch opened its contacts 4 times during a single call for heat. Unlike code 3 (pressure switch did not close at all), code 6 tells you the switch IS closing — the furnace starts up and runs — but then the pressure switch intermittently opens during operation, which the board interprets as a venting safety issue.

This intermittent behavior is typically caused by a partial vent obstruction that allows enough draft for startup but occasionally restricts flow during sustained operation. Wind gusts blowing into the vent termination can momentarily push exhaust back and trigger the switch. A pressure switch hose that is cracked or loosening can also cause intermittent readings. On some installations, the vent run may be borderline — just barely long enough or with too many elbows — so any additional restriction tips the balance.

The system automatically resets after 1 hour, or you can cycle power. But the underlying cause should be addressed or the lockout will keep recurring.

How to Fix It: Check the Exhaust Vent and Intake for Partial Obstructions

What You'll Need

• Flashlight
• Ladder (if needed)
• Gloves

Steps

1. Turn off power AND shut off gas supply Turn the furnace power switch OFF. Turn the gas shutoff valve to OFF (perpendicular to pipe). Wait 5 minutes. If you smell gas at any point, leave the area immediately and call your gas company.
2. Check exterior vent openings thoroughly Go outside and inspect the vent pipe termination(s). Because this code indicates intermittent blockage, look carefully for partial obstructions — a nest that is not fully blocking the pipe, ice buildup on the rim, a screen that is partially clogged with lint, or a vent cap that is damaged or mispositioned. Also note if the vent is positioned where strong wind could blow directly into it.
3. Clear any partial blockages Remove any debris, ice, or partial obstructions. If the vent has a screen or mesh, make sure it is clean and not clogged with lint or ice. If both exhaust and intake pipes exit near each other, make sure they have adequate separation (usually 12 inches minimum) so exhaust is not recirculated into the intake.
4. Check the pressure switch hose inside Inside at the furnace, inspect the small rubber pressure switch hose. A hose that is cracking, softened, or fits loosely on its port can cause intermittent pressure readings. Push connections firmly onto their ports.
5. Restore and test Turn gas back ON. Restore power. Call for heat and let the furnace run a full heating cycle (15-30 minutes) to verify stability.

When to Call a Professional

Contact a licensed HVAC technician if:

• The 6-red-flash code keeps recurring despite clear vent pipes
• The lockout happens specifically during windy conditions (may need a wind-resistant vent cap)
• The inducer motor sounds weak or inconsistent
• The pressure switch hose needs replacement and you are unsure of the correct size
• You suspect the vent run is too long or has too many elbows