Lennox Furnace Error Code 5+5

The 5+5 error code on a Lennox furnace indicates that the 24-volt control circuit voltage has dropped below the level required for safe and reliable operation. The control board monitors this low-voltage circuit continuously and triggers this fault when it detects an undervoltage condition. The furnace will attempt to restart automatically once the voltage rises back to a normal level, so no manual reset is required if the root cause is resolved.

The 5+5 fault itself is a protective response by the furnace control board and is not inherently dangerous. The furnace shuts down operation to prevent unpredictable behavior from components that need stable voltage. The underlying cause, such as a shorted wire or component, should be identified and corrected to prevent repeated faults and potential damage to the transformer or control board over time.

Repair costs vary depending on the root cause. If only a loose wire connection needs attention, the cost may be limited to a service call fee, typically in the range of $75 to $150. Replacing a failed 24V transformer generally costs between $150 and $350 including parts and labor. If the control board itself is faulty, replacement costs can range from $300 to $700 or more depending on the specific board and labor rates in your area.

Some causes of the 5+5 code, such as loose low-voltage wiring connections, can be addressed by a knowledgeable DIYer comfortable working with low-voltage wiring. However, diagnosing and replacing a failing 24V transformer or identifying a shorted component drawing excess current requires a multimeter and familiarity with HVAC electrical systems. If you are not comfortable with electrical diagnostics, contacting a qualified HVAC technician is the safer route.

The Lennox control board is designed to restart automatically once the 24V control circuit voltage returns to a normal operating level, so a manual reset is generally not necessary. If the underlying cause of the low voltage is still present after a reset, the fault will return quickly. Resetting without diagnosing and correcting the root cause is not a lasting solution.