Lennox Furnace Error Code 4 Flashes (RAM)

The 4 Flashes (RAM) error indicates that an open limit condition has been detected. This means the control board has received a signal that the primary limit switch, one of the rollout switches, or a secondary limit has opened due to an overtemperature condition. The furnace shuts down the heating cycle as a safety measure until the root cause of the excess heat is identified and corrected.

The furnace will typically shut itself down when this fault is active, which is the intended safety response. You should not attempt to force the furnace to run through repeated resets if the fault keeps returning, particularly if a rollout switch is the device that tripped. Repeated rollout switch trips can indicate a cracked heat exchanger or combustion issue that poses a carbon monoxide risk.

Repair costs vary widely depending on the cause. If the fix is simply replacing an air filter or resetting a limit switch after clearing a blocked vent, costs can be minimal, typically in the range of $10 to $50 for parts. If the blower motor, inducer motor, or a limit switch itself requires replacement, costs typically range from $150 to $600 or more depending on parts and labor. A cracked heat exchanger represents the most expensive scenario, potentially $500 to $1,500 or higher, or in some cases a full furnace replacement.

Start by replacing the air filter and confirming all supply and return vents throughout the home are open and unobstructed. Next, locate the tripped limit or rollout switch on the furnace and reset it manually if it has a reset button. If the error returns after reset, a technician should inspect the blower motor, flue venting, and burner assembly to find the underlying overtemperature source before the switch is reset again.

Resetting a manually tripped limit or rollout switch may allow the furnace to restart and clear the fault code temporarily. However, if the condition that caused the overtemperature event has not been resolved, the switch will trip again and the error will return. A reset should always be followed by a full heating cycle observation to confirm the fault does not recur.