Lennox Furnace Error Code DS1 On / DS2 Off

This diagnostic code means the 120V main power supply to the furnace has reversed polarity, with the line (hot) and neutral wires connected to the wrong terminals. The furnace control board uses the DS1 and DS2 diagnostic switches to detect this condition. The fix is to swap the line and neutral wires at the 120V field connection point on the furnace after turning off power at the breaker.

Yes, reversed polarity is a genuine electrical safety concern and should be corrected promptly. Beyond the risk to the furnace control board and flame sensing components, reversed polarity means the neutral side of the circuit carries line voltage, which creates shock hazards during servicing. The furnace's diagnostic system flags this condition specifically because it can affect safe operation of the unit.

If the issue is isolated to the furnace field wiring connection, an HVAC technician can typically resolve it in under an hour, putting labor costs in the range of roughly $75 to $150 depending on your area and service provider. No replacement parts are needed in most cases since this is a wiring correction. If the reversed polarity originates at the electrical panel or a junction box, an electrician may need to be involved, which could add to the overall cost.

This repair involves working with 120V wiring, so it should only be attempted by someone comfortable with electrical work and proper lockout procedures. The fix itself is straightforward: shut off the breaker, identify the line and neutral wires at the furnace field connection, swap them to the correct terminals, and restore power. If you are unsure which wire is the hot (line) conductor, use a non-contact voltage tester or multimeter before making any changes, or contact a licensed HVAC technician.

No, simply resetting the furnace will not clear this fault. The DS1 On / DS2 Off condition is detected continuously based on the live electrical wiring, so the code will persist every time the furnace powers up until the line and neutral wires are correctly swapped at the 120V field connection. The fault must be corrected at the source before normal operation can resume.