Coleman Furnace Slow Green Flash
StatusShown as: Green LED on two seconds, off two seconds, repeating
Normal operation. The board is powered and healthy and there is no thermostat call. This is the LED's resting state, not a fault.
Manual wording: “SLOW GREEN FLASH: Normal operation, no thermostat calls.”
Documented for: TG8S, TGLS, TG9S, TM8X, TMLX, TM9X, TM8T, TM9T, TM8V, TM9V
Likely causes, most common first
- Normal standby - no heating, cooling or fan call from the thermostat
What you can safely check yourself
- If the house is cold, check the thermostat is set to HEAT and above room temperature
- Check the thermostat batteries
When to call a technician
No service needed for this code. If the furnace still will not run with the thermostat calling, have a technician check the thermostat wiring.
A Coleman furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation Manual - Residential Gas Furnace, Models TG9S*MP, GG9S*MP (95.5% AFUE Single Stage Multi-position), 364861-UIM-G-1211 (Johnson Controls Unitary Products (Coleman), TG9S*MP, GG9S*MP 95.5% AFUE single-stage multi-position gas furnaces) · checked 2026-08-25
- Installation Manual - Residential Gas Furnace, Models TG8S*MP, TGLS*MP, GG8S*MP, GGLS*MP (Single Stage Multi-position / Low NOx), 356485-UIM-G-1211 (Johnson Controls Unitary Products (Coleman), TG8S*MP, TGLS*MP, GG8S*MP, GGLS*MP 80% AFUE single-stage gas furnaces) · checked 2026-08-25
- Installation Manual - Residential Gas Furnace, Models TM9V*MP (Two Stage Variable Speed ECM), 701379-UIM-B-1211 (Johnson Controls Unitary Products (Coleman), TM9V*MP two-stage variable-speed ECM gas furnaces) · checked 2026-08-25
- Installation Manual - Residential Gas Furnace, Models TM8X*MP, TMLX*MP (Single Stage Multi-position / Low NOx with High-Efficiency Motors), 437642-UIM-F-1211 (Johnson Controls Unitary Products (Coleman), TM8X*MP, TMLX*MP 80% AFUE single-stage gas furnaces with high-efficiency motors) · checked 2026-08-25