Lennox SLP98UHV Error Code df: Defrost Mode
What Does Code df Mean?
The letters df on the SLP98UHV display mean the system is in defrost mode, which applies when the furnace is paired with a heat pump outdoor unit. In cold weather, frost and light ice build up on the outdoor coil, and the system periodically melts it off so the heat pump can keep transferring heat efficiently.
During defrost, the heat pump briefly reverses — the outdoor unit runs in cooling to push hot refrigerant through the outdoor coil and clear the frost. That means the outdoor unit stops sending heat indoors for a few minutes, so on this system the gas furnace burners may fire as backup heat to keep the supply air warm and avoid a cold draft from the vents.
Defrost cycles are completely normal and typically last only a few minutes. They occur more often in cold, damp conditions. No action is needed — the system returns to normal heating automatically once the coil is clear.
What You'll Notice
- The LED shows df while the outdoor unit runs but stops delivering warm air for a few minutes; steam may rise from the outdoor coil as ice melts.
- The furnace burners may fire as backup heat during the cycle so the supply air stays warm, and normal heating resumes on its own.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Heat pump outdoor unit is running a defrost cycle to clear ice buildup on the outdoor coil | Most common | — |
How This Is Diagnosed
Confirm this is normal by simply waiting: a defrost cycle usually ends within a few minutes and the df display returns to normal heating. If the outdoor coil is visibly clearing of frost and heat resumes afterward, the cycle worked as designed and no service is needed.
- Defrost cycles repeat very frequently (for example every 15-20 minutes) over an extended period
- A thick layer of ice remains on the outdoor unit and does not clear after the cycle
Frequently Asked Questions
Why did my heat pump blow cool air and show df?
During a defrost cycle the outdoor unit briefly reverses to melt frost off its coil, so it stops adding heat for a few minutes. On systems with the furnace as backup, the burners fire to keep the supply air warm. This is normal.
How long should defrost last?
Usually only a few minutes. Cold, damp weather makes cycles more frequent. It ends automatically and returns to normal heating.
Is defrost mode damaging my system?
No. It is a built-in, automatic function that protects efficiency by keeping the outdoor coil clear of ice.
✓ Verified against manufacturer service manual — March 2026