Lennox Furnace Error Code 4+8

The 4+8 fault code means the control board (100870) has detected line voltage below 90 volts AC at the furnace. The furnace shuts down to prevent damage from operating under insufficient voltage. This fault points to a problem with the electrical supply rather than an internal furnace component. Measuring voltage at the furnace with a multimeter is the correct first diagnostic step.

The furnace's control board triggers this code specifically to prevent unsafe operation under low-voltage conditions, so the furnace locking out is itself a protective response. However, persistent low voltage in a home can indicate a serious electrical problem such as a failing neutral connection or utility issue, which can pose risks to other appliances and should be investigated promptly. Contact your utility company if voltage is low throughout the home.

Resetting the furnace may temporarily clear the fault, but the code will return as soon as the control board detects line voltage below 90V again during operation. The underlying voltage problem must be identified and corrected for the fault to clear permanently. Repeated resets without fixing the cause can mask the problem and may lead to component stress or damage over time.

Some causes, such as a utility voltage problem or a tripped breaker, can be identified without special tools, but diagnosing low voltage conditions safely requires a multimeter and basic electrical knowledge. If the problem is found to be in your home's wiring, panel connections, or circuit breaker, a licensed electrician should perform the repair. Do not attempt to work inside the electrical panel unless you are qualified to do so.

Costs vary widely depending on the root cause. If the utility is delivering low voltage, there may be no cost to you for the repair itself. A failing circuit breaker replacement typically runs in the range of $150 to $350 through an electrician. Wiring repairs or panel work can range from $200 to over $1,000 depending on the scope. If the issue is simply a loose connection, costs may be minimal.