Lennox SLP98UHV Error Code U: Discharge Air Temperature
What Does Code U Mean?
The letter U on the Lennox SLP98UHV display indicates the discharge air temperature diagnostic readout. This appears when cycling through the diagnostic menu using the push button on the control board. It shows the temperature of the air leaving the furnace and entering the supply ductwork.
The discharge air temperature is one of the key measurements technicians use to verify a furnace is operating correctly. During heating, the supply air temperature should typically be 40-70 degrees Fahrenheit above the return air temperature, depending on the firing rate and airflow. A reading that is too high can indicate restricted airflow, while a reading that is too low might suggest a gas pressure problem or incorrect firing rate.
This is an informational display only and requires no action from the homeowner. The display will continue cycling through other diagnostic items or return to the idle state.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Diagnostic push button pressed to view discharge air temperature | Most common | — |
When to Call a Professional
Contact a licensed HVAC technician if:
- You are concerned that the air from your vents is not warm enough or is excessively hot
✓ Verified against manufacturer service manual — March 2026