Coleman Furnace Continuous Red Flash
WarningShown as: Red LED flashing on and off continuously with no pauses
Twinning error on the TM9T / TM8T 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: TM9T, TM8T, TMLT
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 Coleman furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Manual - Residential Gas Furnace, Models TM9T*MP (96% AFUE Two Stage Multi-position), 505309-UIM-D-0511 (Johnson Controls Unitary Products (Coleman), TM9T*MP 96% AFUE two-stage PSC gas furnaces (TM8T*MP / TMLT*MP manual 521271-UIM-D-1211 carries the same table)) · checked 2026-08-25