Lennox SLP98UHV Error Code d: Dehumidification Mode
What Does Code d Mean?
The lowercase letter d in the SureLight control's menu reports the dehumidification-mode airflow. It shows up as part of the diagnostic push-button cycle, with the LED displaying the blower CFM used during dehumidification followed by a brief pause before repeating.
This is purely informational. The SLP98UHV can enter a dehumidification mode when it is paired with a communicating iComfort thermostat that has humidity control. In that mode the blower deliberately runs slower than in normal cooling so the evaporator coil stays colder and pulls more moisture out of the air; the d readout shows that reduced airflow setting.
No action is required. The display continues cycling through other diagnostic items or returns to idle on its own. If your home actually feels too humid, that is a thermostat configuration matter for a technician rather than a fault signalled by this readout.
What You'll Notice
- The lowercase letter d appears on the LED followed by a CFM value, then pauses and repeats.
- The furnace and blower keep operating in whatever mode they were in; the readout is purely a display.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Diagnostic push button pressed to view dehumidification settings | Most common | — |
How This Is Diagnosed
Confirm this is a normal readout by checking that it only shows while the diagnostic push-button menu is being viewed. Let the menu advance — a new item every five seconds — past the d entry, or wait for it to return to idle; normal operation resuming confirms there is no problem to fix.
- Your home feels persistently humid even when the system runs in dehumidification mode
- You want the dehumidification airflow reviewed because comfort is poor
Frequently Asked Questions
What is the 'd' code on my Lennox SLP98UHV?
It is the diagnostic readout for the airflow (CFM) the blower uses during dehumidification mode. It is information, not an error.
Does my furnace dehumidify on its own?
The SLP98UHV can lower blower speed to improve moisture removal when paired with a communicating iComfort thermostat that has humidity control. The d readout shows that reduced-airflow setting.
Do I need to do anything about the d display?
No. It clears when the diagnostic menu times out. If your humidity is genuinely too high, that is a thermostat configuration question for a technician, not a fault indicated by this readout.
✓ Verified against manufacturer service manual — March 2026