Lennox Furnace Error Code 5+6

The 5+6 fault code on a Lennox furnace with control board 100869 indicates low secondary voltage on the 24VAC control circuit. This is the low-voltage power supply that runs the thermostat, control board, and components like the gas valve. The furnace flags this condition because insufficient control voltage can cause unreliable or unsafe operation. The root cause is typically a failing transformer, an overloaded secondary circuit, or a shorted low-voltage component.

Low 24VAC voltage can cause control components to behave unpredictably, which is why the furnace shuts down rather than continuing to run. It is not an immediately dangerous condition in the way a gas leak or carbon monoxide issue would be, but the furnace should not be forced to run while the fault is present. Have the underlying cause diagnosed and corrected before restoring normal operation.

Resetting the furnace by cycling power may temporarily clear the fault display, but the code will return if the underlying low-voltage condition has not been resolved. The 5+6 code is triggered by an active electrical condition, not a one-time event, so it will continue to appear until the failing transformer, shorted component, or wiring issue is corrected. A reset alone is not a repair.

Some diagnostic steps, such as inspecting wiring connections for corrosion or looseness, are within reach of a knowledgeable DIYer with a multimeter. However, measuring live transformer output voltage and isolating shorted components requires comfort working around electrical circuits. Replacing the transformer or control board 100869 involves handling line-voltage wiring and is generally best left to a qualified HVAC technician.

Repair costs vary depending on the root cause. A replacement transformer typically runs in the range of $50 to $150 for the part, plus labor. If control board 100869 is the issue, board replacement costs are generally in the range of $150 to $400 or more for the part alone. A diagnostic service call from an HVAC technician to identify the specific cause usually adds $75 to $150 to the total cost.