Lennox Furnace Error Code 4+5

The 4+5 flash code on a Lennox furnace with control board 100869 means the Watchguard circuit has detected that the high-limit switch stayed open for more than three consecutive minutes. The high-limit switch is a safety device that shuts off the burners when the furnace overheats. When it cannot reset within that three-minute window, the board locks out the burner and displays this code to indicate a persistent overheating or limit-circuit fault.

Yes, this code should be taken seriously. The high-limit switch is a primary safety device that prevents the heat exchanger from overheating. Repeated overheating cycles can crack the heat exchanger over time, which can allow combustion gases to enter the living space. Do not simply reset and ignore this code without identifying and correcting the underlying cause.

Repair costs depend entirely on the root cause. A filter replacement costs only a few dollars and is a straightforward DIY task. A failed high-limit switch typically runs in the range of $100 to $300 installed, depending on labor rates in your area. A blower motor replacement is a more involved repair and can range from $300 to $700 or more depending on the motor type and local labor costs. Getting a diagnostic service call first is the best way to get an accurate estimate.

Some causes of this code are DIY-friendly. Start by turning off the furnace, replacing the air filter, and confirming all supply and return registers are fully open. Check whether any manually-reset limit or rollout switches have a small button that has popped out and needs to be pressed back in. If the blower motor is not running or the filter and airflow checks do not resolve the issue, the repair moves into component testing that typically requires a qualified technician.

A power reset may temporarily clear the lockout and allow the furnace to attempt operation again, but if the root cause has not been addressed, the limit switch will continue to trip and the 4+5 code will return. Resetting is useful for testing after completing a repair, but it is not a fix on its own. Repeated resets without resolving the underlying issue can accelerate heat exchanger wear.