Lennox HVAC Error Code 355

Error code 355 on a Lennox air handler indicates a problem with the fifth stage electric heat circuit. The system has determined that the stage 5 pilot relay coil did not energize or its contacts did not close as expected during a heating call. In response, the air handler limits itself to first stage operation only until the fault is corrected.

Yes, your system will continue to operate, but only on first stage electric heat. This means reduced heating output compared to normal full-capacity operation. In mild weather this may be sufficient, but during very cold conditions your home may not reach the desired temperature without all stages functioning.

Repair costs vary depending on the root cause. Replacing a pilot relay is generally a moderately priced repair, with parts typically ranging from $20 to $80 and labor costs adding to the total. If the control board is found to be at fault, costs will be higher, generally ranging from $200 to $600 or more including labor. A qualified HVAC technician can diagnose the specific faulty component before committing to parts costs.

Code 355 is self-clearing once the control board successfully detects that the stage 5 relay has closed. Simply resetting the system without fixing the underlying problem will not permanently clear the fault, as it will return the next time fifth stage heat is called. The relay coil, contacts, and associated wiring must be inspected and repaired or replaced to resolve the issue.

The fault itself represents a component failure rather than an immediate safety hazard, since the system is designed to fall back to reduced operation rather than shut down entirely. However, a failed relay or wiring fault in an electric heat circuit should be professionally inspected promptly, as deteriorating electrical components can pose risk if left unaddressed for extended periods.