Lennox HVAC Error Code 304

Error code 304 means the Y2 relay on the air handler control board has failed. The control board commanded the Y2 pilot relay to energize for second-stage cooling, but the relay coil did not energize or the relay contacts did not close as expected. This prevents the system from operating in second-stage cooling mode. The fault clears after a reset once a valid Y2 input is sensed by the new control board.

A 304 fault disables second-stage cooling, meaning the system will likely still operate in first-stage mode and provide some cooling. However, on very hot days, first-stage capacity alone may not be enough to maintain comfortable temperatures. Running the system long-term with this fault also puts additional runtime stress on first-stage components. It is best to have the fault repaired promptly rather than relying on reduced capacity indefinitely.

A reset will temporarily clear the 304 alarm, but the fault will return if the underlying Y2 relay problem has not been corrected. The alarm is designed to clear permanently only after a reset is performed AND the control board successfully senses a valid Y2 input, confirming the relay is now working. Simply cycling power without replacing a faulty board will not resolve the issue.

Basic checks like inspecting wiring connections at the Y2 terminal and verifying 24VAC control voltage are within reach for experienced DIYers. However, if the control board itself has failed, replacing it involves working inside the air handler with live electrical components and requires proper identification of the correct replacement board. Most homeowners will want a qualified HVAC technician to perform the control board replacement.

If the fix requires a control board replacement, parts and labor costs typically range from around $200 to $600 or more depending on the specific air handler model and local labor rates. If the fault turns out to be a simple wiring issue, the repair cost will be considerably lower, covering only a diagnostic service call. Getting a diagnosis from a technician before assuming the board has failed can help avoid unnecessary parts costs.