Lennox SLP98UHV Error Code df: Defrost Mode
What Does Code df Mean?
The letters df on the Lennox SLP98UHV display indicate that the system is in defrost mode. This only applies when the furnace is paired with a heat pump outdoor unit. During cold weather, frost and ice naturally build up on the outdoor unit's coil, and the system periodically runs a defrost cycle to melt the ice and maintain efficient operation.
During defrost, the heat pump temporarily reverses its operation — the outdoor unit switches to cooling mode to send hot refrigerant through the outdoor coil and melt the frost. This means the outdoor unit stops providing heat to the home for a few minutes. The furnace may activate its burners as backup heat during this period to prevent cold air from blowing through the vents.
Defrost cycles are completely normal and typically last 2 to 10 minutes. They happen more frequently during cold, humid weather. No action is needed — the system will automatically return to normal heating mode once the defrost cycle completes.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Heat pump outdoor unit is running a defrost cycle to clear ice buildup on the outdoor coil | Most common | — |
When to Call a Professional
Contact a licensed HVAC technician if:
- Defrost cycles are happening extremely frequently (every 15-20 minutes)
- The outdoor unit has a large ice buildup that does not clear after a defrost cycle
✓ Verified against manufacturer service manual — March 2026