Lennox Furnace Error Code E348

E348 means the furnace integrated control board is not producing a 24-volt output on the Y2 terminal when communicating with a non-communicating (conventional) outdoor unit. Y2 is the signal that commands second-stage cooling from a two-stage outdoor unit. The fault code identifies the Y2 relay or its associated circuit on the control board as having failed. The system can still operate in first-stage cooling via Y1, but full cooling capacity is unavailable.

E348 does not create an immediate safety hazard. The system will continue to operate in first-stage cooling and normal heating modes. However, running with a failed Y2 stage means the system cannot meet high cooling demand efficiently, which can cause extended run times, reduced comfort, and added wear on the outdoor unit. It is best to have the issue diagnosed and repaired promptly rather than running the system indefinitely in this condition.

A knowledgeable DIYer can check the Y2 terminal wiring at the control board and at the outdoor unit for loose or corroded connections, and can verify that the thermostat is wired and configured for two-stage cooling. However, confirming a failed relay on the integrated control board requires measuring 24-volt output at the Y2 terminal during a live cooling demand, which involves working with an energized system. Replacing the integrated control board is a moderate-difficulty repair but requires correct board identification and safe shutdown procedures. When in doubt, a licensed HVAC technician should perform the diagnosis and repair.

A reset will clear E348 temporarily, and the code clears when a Y1 input signal is sensed after the reset. However, if the Y2 relay on the integrated control board has mechanically failed, the fault will return as soon as the thermostat demands second-stage cooling again. Repeated recurrence after reset is a strong indicator that the control board or its relay requires physical repair or replacement.

If the repair requires replacing the integrated control board, costs typically range from around 300 dollars to over 700 dollars depending on the specific board, labor rates in your area, and whether the system is under warranty. If the fault is caused by a loose or damaged wire at the Y2 terminal, the repair cost is much lower, often limited to a service call fee in the range of 80 to 150 dollars. A technician should confirm the failed component before parts are ordered.