York Furnace Steady On Red (YP9C Modulating)
Hard lockoutShown as: Red LED lit solid, not flashing
A control fault has been detected, or 115 volts is present without 24 volts.
Manual wording: “Steady On Red - Control fault has been detected or there is 115 volts present without 24 volts... If there is both 24 and 115 voltage to the board, replace the control board.”
Documented for: YP9C, YPLC modulating furnaces
Likely causes, most common first
- Missing 24 volt supply to the board (blown control fuse or failed transformer)
- Failed control board
What you can safely check yourself
- Turn the furnace off at the service switch for a minute, then back on once
When to call a technician
If both 24V and 115V are present and the LED is still solid red, the board has to be replaced. Voltage checks and board replacement are technician work.
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