Lennox SLP98UHV Error Code E 348: No 24V Output Y2 to C
What Does Code E 348 Mean?
In non-communicating mode the Lennox SLP98UHV drives the outdoor air conditioner or heat pump through 24-volt relay outputs on the SureLight Variable Capacity Integrated Control. E 348 means the board tried to energize the Y2 relay — the output between the Y2 and C terminals that brings on second-stage (high-stage) cooling — but no 24 volts were produced. Either the relay contacts did not close or the relay coil did not energize.
On a two-stage outdoor unit, Y2 is what lets the system step up to full cooling capacity on hot days. If that output never appears, the system may still cool weakly on first stage but can never ramp up to keep pace with demand, or it may not cool at all if the failure is more general.
E 348 is part of the non-communicating relay-output family. It sits alongside E 347 (no 24-volt output on Y1 to C, the first-stage cooling relay) and E 349 (no 24 volts between R and O, the missing R-to-O jumper needed for heat-pump reversing-valve operation). A technician seeing several of these together will focus on the board's relay outputs and low-voltage supply.
What You'll Notice
- The air conditioner cools weakly and never seems to reach full capacity on hot days
- The outdoor unit runs but the home stays warm during peak cooling demand
- Cooling starts on low stage but never steps up to high stage
- On some systems the outdoor unit does not come on for cooling at all
- The thermostat runs a long cooling call without satisfying the setpoint
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Y2 relay on control board has failed | Most common | ✗ Call a pro → |
How This Is Diagnosed
The technician first confirms the homeowner-safe basics — thermostat set to cool, outdoor unit clear of debris and powered at its disconnect. The remaining checks belong to the pro.
They meter across the Y2 and C terminals during a call for second-stage cooling. If no 24 volts appears while stage-2 is clearly being requested, the Y2 relay on the control board is not closing. After confirming the low-voltage supply and wiring are intact, the failed relay is isolated, which on this integrated control normally 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 system cools only weakly and never ramps to full capacity on hot days
- E 348 appears with E 347 or E 349, indicating the board's relay section
- Second-stage cooling is needed and the output is missing
- The Y2 relay output or board low-voltage circuit needs professional testing
Frequently Asked Questions
Why does my Lennox SLP98UHV cool a little but never keep up on hot days?
E 348 means the board is not energizing the Y2 relay for second-stage cooling, so the outdoor unit cannot step up to full capacity. It may still run on first stage, which cools slowly.
Is E 348 the same as E 347?
They are closely related. E 347 is a missing first-stage (Y1) cooling output, while E 348 is a missing second-stage (Y2) output. Both are relay-output failures on the furnace control board in non-communicating mode.
Does E 348 mean my compressor is bad?
Usually not. The fault is that the furnace board never sent the second-stage start signal. The compressor itself is typically fine, though a technician should confirm.
✓ Verified against manufacturer service manual — March 2026