York Furnace Continuous Red Flash
WarningShown as: Red LED flashing on and off continuously with no pauses
Twinning error on the TM8Y / TM9Y (and TM8T / TM9T) boards: incorrect 24V phasing between two twinned furnaces, or no power to the twinned unit.
Manual wording: “CONTINUOUS RED FLASH: Twinning error, incorrect 24V phasing or no power to twinned unit. Check twinning wiring. Confirm that both twinned units have power.”
Documented for: TM8Y, TM9Y (and the TM8T / TM9T two-stage PSC chassis)
Likely causes, most common first
- Incorrect 24V phasing between twinned furnaces
- No power to the twinned unit
- Twinning wiring error
What you can safely check yourself
- Check the breaker and service switch for the second (twinned) furnace are on
When to call a technician
Have a technician verify the twinning wiring and 24V transformer phasing between the two furnaces.
A York furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Manual - Two Stage Standard ECM Residential Gas Furnaces, TM8Y (80% AFUE Multi-position), 5052632-UIM-D-0217 (Johnson Controls Unitary Products (York), TM8Y*MP two-stage standard ECM 80% AFUE gas furnaces (TM9Y manual 5052617-UIM-G-0817 carries the same table)) · checked 2026-08-25