Lennox SLP98UHV Error Code E 347: No 24V Output Y1 to C
What Does Code E 347 Mean?
When the Lennox SLP98UHV is wired in non-communicating mode, the SureLight Variable Capacity Integrated Control switches the outdoor air conditioner or heat pump using ordinary 24-volt relay outputs on its terminal strip. E 347 means the control tried to energize the Y1 relay — the output between the Y1 and C terminals that starts first-stage (low-stage) cooling — but no 24 volts appeared. The board detected that its own pilot relay contacts did not close or the relay coil did not energize.
Because the furnace board is the component that closes this circuit, the fault is almost always on the board itself rather than in the outdoor unit. The outdoor compressor never receives a start signal, so cooling simply never begins even though the thermostat is calling for it.
E 347 belongs to the same non-communicating relay-output family as E 348 (no 24-volt output on Y2 to C, the second-stage cooling relay) and E 349 (no 24 volts between R and O, meaning the R-to-O jumper needed for heat-pump reversing-valve operation is missing). If more than one of these appears together, a technician will look closely at the board's relay section and its low-voltage supply.
What You'll Notice
- The air conditioner or heat pump will not turn on when the thermostat calls for cooling
- The outdoor unit stays completely off during a cooling call while heating still works normally
- Air from the supply vents is room-temperature or warm during a cooling call
- The thermostat shows it is cooling, but the home never gets cooler
- Cooling fails on the very first (low) stage, so the system never ramps up
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Y1 relay on control board has failed | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician starts with the homeowner-safe checks: confirming the thermostat is actually set to cool and that the outdoor unit is not blocked by debris or turned off at its disconnect. From there the work moves to the board, which the homeowner should leave to the pro.
The technician places a meter across the Y1 and C terminals during a cooling call. If there is no 24 volts there while the thermostat is clearly calling for stage-1 cooling, the Y1 relay on the control board is not closing. They confirm the low-voltage supply and wiring are good and then isolate the failed relay, which on this integrated control usually means replacing the board.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The outdoor unit never starts on a cooling call even though heating works
- E 347 appears together with E 348 or E 349, pointing to the board's relay section
- Cooling is needed and the system is stuck with no first-stage output
- The furnace board's Y1 relay or low-voltage circuit needs to be tested or replaced
Frequently Asked Questions
Why is my heat working but the AC will not cool on my Lennox SLP98UHV?
E 347 means the furnace control board is not energizing the Y1 relay that starts first-stage cooling, so the outdoor unit never receives a start signal. Heating uses a different part of the board, which is why it can still work normally.
Is E 347 a problem with my outdoor AC unit or the furnace?
It is almost always the furnace control board. The board failed to close the Y1 output that tells the outdoor unit to start, so the outdoor unit itself is usually fine.
Can I fix E 347 myself?
No. Diagnosing and repairing a failed relay output involves low-voltage testing and usually control-board replacement, which should be done by a qualified HVAC technician.
✓ Verified against manufacturer service manual — March 2026