York Furnace 1 Green Flash (YP9C Modulating)
StatusShown as: One green flash, two-second pause, repeating
Normal operation with no call for heat. Standby.
Manual wording: “One Green Flash - Normal Operation with no call for heat.”
Documented for: YP9C, YPLC modulating furnaces
Likely causes, most common first
- Normal standby
What you can safely check yourself
- If the house is cold, check the thermostat is set to HEAT and above room temperature
When to call a technician
No service needed.
A York furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Manual - Modulating ECM Residential Gas Furnace with ECM Motor, Models YP9C*C / TP9C*C / CP9C*C / LP9C*C (Up to 98% Modulating Multi-position), 1083292-UIM-A-0114 (Johnson Controls Unitary Products (York), YP9C*C (York Affinity modulating), and the identical TP9C / CP9C / LP9C chassis) · checked 2026-08-25