411
Moderate
Lennox · HVAC Systems (AC, Furnaces & Heat Pumps)
Communication Fault Between Indoor and Outdoor Units
Quick Answer
Error code 411 on your Lennox HVAC Systems (AC, Furnaces & Heat Pumps) means: Communication Fault Between Indoor and Outdoor Units. This is usually fixable — see the step-by-step guide below.
Likely Causes
How to Fix It
1
Verify all communication wiring connections are tight at both indoor and outdoor unit terminals
2
Test the communication wire for continuity between the two units
3
Check for proper voltage on the communication bus (per Lennox installation manual)
4
Inspect the wire run for physical damage, rodent chew marks, or UV degradation
5
If wiring tests good, a Lennox-trained HVAC technician should diagnose the control boards
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Communication wire
Indoor control board
Outdoor control board
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