Lennox SLP98UHV Error Code E 347: No 24V Output Y1 to C
What Does Code E 347 Mean?
Error code E347 on the Lennox SLP98UHV indicates that there is no 24-volt output between the Y1 and C terminals with a non-communicating outdoor unit. The Y1 relay (Stage 1 cooling relay) on the control board has failed — either the relay contacts did not close, or the relay coil did not energize when commanded.
The Y1 signal is what tells the outdoor air conditioner or heat pump to turn on for first-stage cooling. Without this signal, the outdoor unit will not start even though the thermostat is calling for cooling. The furnace blower may still run, but no cooling will occur.
This is an internal control board failure. The relay is a component soldered onto the board and cannot be individually replaced in the field. A technician will need to verify the fault by testing for 24V at the Y1 terminal and, if confirmed, replace the entire control board.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Y1 relay on control board has failed | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Your air conditioner or heat pump does not turn on when the thermostat calls for cooling
- The furnace blower runs but no cool air comes out
- The outdoor unit is completely silent when it should be running
✓ Verified against manufacturer service manual — March 2026