Lennox Furnace Code E312
Warning
Restricted air flow in cooling or continuous fan mode. The blower is delivering less air than its cfm setting because the variable speed motor has hit its built-in speed and torque limits.
Manual wording: “Restricted air flow in cooling or continuous fan mode is lower than cfm setting. Indoor blower is running at a reduced cubic feet per minute (Cutback Mode).”
Documented for: SLP98UHV series
Likely causes, most common first
- Dirty air filter
- Restricted or undersized ductwork pushing total external static pressure above 0.8 inches water column
- 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, static pressure has to be measured by 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