Rheem, a furnace and home appliance manufacturer based in California, produces a line of furnaces for home use. Some Rheem models — particularly newer furnaces — have an integrated furnace control inside them that flashes a warning LED light when there is a problem with operation. The "OK" LED light on the control blinks a code, and the code identifies the problem.
One blink of the light with a two-second pause before the next blink signals a lockout for one hour. During lockout, a Rheem furnace will not turn on or function. Lockout is a safety feature that can be caused by more than one problem; on some Rheem models, lockout is caused by a failed ignition sequence. A problem during the ignition sequence can be a safety hazard — a gas leak, for example — so a professional should check the furnace even though turning off power to the furnace off and restarting it might end the lockout.
Two blinks of the light with a two-second pause means the pressure switch is open. The pressure switch monitors the furnace's ventilation. Whether the switch open or closes depends on the amount of pressure in the furnace; Rheem sets pressure defaults during the manufacturing process. While the switch should be open at the start of the ignition process, it should close at some point — depending on the model — before the process continues. If it doesn't close, the error light will blink. Various conditions can cause a pressure switch to open, including a defect with the switch itself or a blocked vent.
Three or Four Blinks
Three LED blinks with a two-second pause indicates the limit switch is closed. A limit switch is a safety feature directly wired into the furnace; it stops a furnace from overheating if air isn't properly flowing through the heat exchanger. If the switch is closed, debris might be clogging the filter or the blower fan housing.
An LED that blinks four times followed by a two-second pause is signaling the pressure switch is closed when it should be open. The switch itself might be stuck, or the tubing that attaches the switch to the furnace's draft inducer might be blocked.
Additional Light Codes
Some Rheem models have "twinning" capabilities. If there's any fault with the setup, the LED will blink five times, followed by a two-second pause. Twinning is when two Rheem furnaces of the same make and model are being used together on the same ductwork and thermostat. Twinned furnaces are usually used if the area being heated needs more heat and air circulation than one furnace can provide but the installation options, connections or existing ductwork don't allow for a larger furnace.