York Furnace 4 Red Flashes (YP9C Modulating)
Soft lockoutShown as: Four red flashes, two-second pause, repeating
High limit switch open, defective temperature sensor, or the 24 volt fuse is open. Usually an airflow restriction is making the furnace overheat.
Manual wording: “Four Red Flashes - High limit switch open or defective temperature sensor or 24 volt fuse is open. This may be caused by a dirty air filter, improperly sized duct system, faulty blower motor, restricted circulating airflow an open fuse on the control board.”
Documented for: YP9C, YPLC modulating furnaces
Likely causes, most common first
- Dirty air filter
- Improperly sized duct system
- Faulty blower motor
- Restricted circulating airflow
- Open fuse on the control board
What you can safely check yourself
- Check and replace the air filter
- Open all supply registers and clear the return grille
- Check that the furnace door panel is fully seated
When to call a technician
If the code returns with a clean filter and open vents, have a technician check the blower, ductwork and the board fuse. Never bypass the limit switch.
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