Lennox HVAC Error Code 354

Error code 354 on a Lennox air handler means the fourth stage electric heat relay has a fault: either the relay coil did not energize or the relay contacts did not close when fourth stage heat was called for. The system responds by limiting operation to first stage heat only until the issue is resolved. The fault clears automatically once the control board detects the stage 4 relay functioning correctly.

The system is designed to continue operating on first stage heat when error code 354 is active, which makes it generally safe to use in the short term. However, reduced heating capacity may result in inadequate heating during very cold weather. The fault should be diagnosed and repaired promptly to restore full system performance and prevent additional strain on remaining components.

The primary diagnostic step is to check the stage 4 relay coil and contacts. A technician will measure coil resistance to confirm the winding is intact and verify that control voltage is present at the coil terminals during a call for fourth stage heat. Contact continuity is also tested to confirm the contacts are closing properly. If the coil and contacts pass inspection, the wiring between the control board and relay is inspected for loose connections, corrosion, or damage before evaluating the control board itself.

Error code 354 is not cleared by a simple manual reset. The alarm clears automatically once the system detects that the stage 4 relay is operating properly. Cycling power may temporarily clear the display, but the fault will return the next time a call for fourth stage heat is made if the underlying relay or wiring problem has not been corrected.

Repair costs depend on the failed component. Replacing a stage 4 relay typically ranges from around $75 to $250 in parts and labor, depending on the specific relay used in your unit. If the control board has failed and requires replacement, costs can range from $300 to $700 or more. Loose or corroded wiring connections can often be corrected during a standard diagnostic service call, which generally runs between $80 and $150.