York Furnace Continuous Green Flash
WarningShown as: Green LED flashing on and off continuously with no pauses
Twinning error. Two furnaces are wired together on a common duct system and the 24V control power is out of phase, or one of the twinned units has lost power.
Manual wording: “CONTINUOUS GREEN FLASH: Twinning error, incorrect 24V phasing or no power to twinned unit. Check twinning wiring. Confirm that both twinned units have power.”
Documented for: TG8S, TG9S and other twinnable UPG single-stage furnaces. On TM8Y / TM9Y the same twinning fault is shown as a continuous RED flash instead.
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
Twinning phasing is a wiring job. A technician needs to verify the 24V transformer phasing and twinning harness between the two furnaces.
A York furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation Manual - Single Stage PSC Residential Gas Furnaces, TG9S and GG9S Series (95.5% AFUE Multi-position), 1083287-UIM-A-0114 (Johnson Controls Unitary Products (York), TG9S*MP, GG9S*MP single-stage PSC 95.5% AFUE gas furnaces) · checked 2026-08-25
- Installation Manual - Single Stage PSC Residential Gas Furnaces, TG8S*MP / TGLS*MP / GG8S*MP / GGLS*MP (80% AFUE Multi-position / Standard Low NOx), 998491-UIM-B-1013 (Johnson Controls Unitary Products (York), TG8S*MP, TGLS*MP, GG8S*MP, GGLS*MP 80% AFUE single-stage gas furnaces) · checked 2026-08-25