Lennox SLP98UHV Error Code U: Discharge Air Temperature
What Does Code U Mean?
The letter U in the SureLight control's menu reports the discharge air temperature — the temperature of the heated air leaving the furnace and entering the supply ductwork. It appears while cycling the diagnostic push button, with the LED showing U followed by the current temperature value.
Discharge air temperature is one of the key numbers a technician uses to confirm the furnace is operating correctly. On this modulating unit it changes as the furnace ramps up or throttles down, and it is judged against the temperature-rise range printed on the unit's rating plate rather than a single fixed number. A reading above that range can point to restricted airflow, while a reading below it can suggest a gas input or firing-rate issue.
This is an informational display only and requires no action from the homeowner. The display continues cycling through other diagnostic items or returns to the idle state on its own.
What You'll Notice
- The LED shows the letter U followed by a temperature value representing the air leaving the furnace into the supply duct.
- The furnace keeps running normally while this value displays, and it changes as the furnace heats up or modulates.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Diagnostic push button pressed to view discharge air temperature | Most common | — |
How This Is Diagnosed
This is a live sensor readout, so confirm it is normal by checking that the furnace is otherwise heating properly and that the discharge temperature falls within the temperature-rise range printed on the unit's rating plate. Letting the diagnostic menu time out returns the display to its normal state.
- The air from your vents feels too cool, or uncomfortably hot, and you want the temperature rise checked
- The discharge reading appears far outside the temperature-rise range on the unit's rating plate
Frequently Asked Questions
What is the U reading on my Lennox furnace?
U is the discharge (supply) air temperature — the temperature of the heated air leaving the furnace. It is a diagnostic readout technicians use to verify proper operation, not an error.
What should the discharge temperature be?
It should fall within the temperature-rise range printed on your furnace's rating plate, which varies by model and installation. A technician compares the reading to that range rather than to a single fixed number.
Do I need to act on the U readout?
No. It is informational and clears when the diagnostic menu times out. It only matters if the furnace is also heating poorly.
✓ Verified against manufacturer service manual — March 2026