Lennox Furnace Code E311
Warning
The furnace has reduced its heat rate to match the airflow the blower can actually deliver. The blower is in cutback mode because airflow is restricted.
Manual wording: “Heat rate reduced to match indoor blower air flow. Replace filter or repair duct restriction. Furnace blower in cutback mode due to restricted airflow.”
Documented for: SLP98UHV series
Likely causes, most common first
- Dirty air filter
- Restricted or undersized ductwork
- Closed or blocked registers
What you can safely check yourself
- Check the air filter and replace it if it is dirty
- Make sure supply and return registers throughout the house are open and unblocked
- Check that the return grille is not covered
When to call a technician
The manual's action is to replace the filter if needed or repair and add ductwork. If a clean filter does not clear it, the duct system needs a technician.
A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- SLP98UHV Service Literature (Corp. 1029-L7), Table 14 - LED 7 Segment Status / Error Code and Integrated Control Diagnostic Codes (Lennox Industries Inc., SLP98UHV series variable-capacity communicating gas furnace (66,000-132,000 Btuh)) · checked 2026-08-25