York Furnace 1 Red Flash (YP9C Modulating)
Safety faultShown as: One red flash, two-second pause, repeating
Flame is present with no power being supplied to the gas valve - gas is reaching the burners when it should not be.
Manual wording: “One Red Flash - Flame is present with no power being supplied to gas valve. This can be caused by a gas valve that is slow to close or that leaks gas through to the burners.”
Documented for: YP9C, YPLC modulating furnaces
Likely causes, most common first
- Gas valve slow to close
- Gas valve leaking gas through to the burners
What you can safely check yourself
- If you smell gas, leave the house and call the gas utility from outside
When to call a technician
Call a licensed technician right away. A leaking gas valve is a safety hazard and only a professional can test and replace it.
A York furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Manual - Modulating ECM Residential Gas Furnace with ECM Motor, Models YP9C*C / TP9C*C / CP9C*C / LP9C*C (Up to 98% Modulating Multi-position), 1083292-UIM-A-0114 (Johnson Controls Unitary Products (York), YP9C*C (York Affinity modulating), and the identical TP9C / CP9C / LP9C chassis) · checked 2026-08-25