York Furnace Slow Green Flash
StatusShown as: Green LED on two seconds, off two seconds, repeating
Normal operation. The control board is powered and healthy and there is no call from the thermostat. This is the LED's resting state, not a fault.
Manual wording: “SLOW GREEN FLASH: Normal operation, no thermostat calls.”
Documented for: TG8S, TG9S, TM9E, TM9V, TM8Y, TM9Y and other UPG single/two-stage boards
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 for heat, have a technician check the thermostat wiring.
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
- Installation Manual - Single Stage ECM Residential Gas Furnaces, TM9E Series (95% AFUE Multi-position), 1083163-UIM-A-0214 (Johnson Controls Unitary Products (York), TM9E*MP single-stage ECM 95% AFUE gas furnaces) · checked 2026-08-25
- Installation Manual - Two Stage Variable Speed ECM Residential Gas Furnaces, TM9V Series (96% AFUE Multi-position), 1083294-UIM-A-0114 (Johnson Controls Unitary Products (York), TM9V*MP two-stage variable-speed ECM 96% AFUE gas furnaces) · checked 2026-08-25