York YP9C four red flashes: limit, sensor, or fuse

What Does Error 4 red flashes Mean?

On the documented York YP9C control, four red flashes can mean an open high-limit switch, a defective temperature sensor, or an open 24-volt fuse. Confirm that the LED is red, not amber, before using that definition. Four amber flashes describes reduced heating capacity from restricted circulating airflow. Applies to: York YP9C, TP9C, CP9C, and LP9C furnaces covered by the cited manual This page covers the YP9C family and the TP9C, CP9C, and LP9C models named in the same York manual. The color is part of the code, and other York controls may assign a different meaning to four flashes. Owner-safe checks: Record the flash color and count through two complete groups without removing a furnace door. | With heating off, inspect the homeowner-access filter and confirm ordinary return and supply grilles are clear. | Note whether the thermostat display remains powered and whether the blower continues after the burners stop. Qualified technician: Fuse diagnosis, sensor testing, temperature-rise measurement, blower assessment, and safety-circuit work belong to qualified gas-furnace service. | Never bypass a limit or install a larger fuse. Either action removes the protection York intended the control to enforce.