421
Moderate
Lennox · HVAC Systems (AC, Furnaces & Heat Pumps)
Electronic Air Cleaner (EAC) Fault
Quick Answer
Error code 421 on your Lennox HVAC Systems (AC, Furnaces & Heat Pumps) means: Electronic Air Cleaner (EAC) Fault. This is usually fixable — see the step-by-step guide below.
Affected Models
Applies to all models
Likely Causes
How to Fix It
1
Remove the electronic air cleaner cells and wash them with warm water and mild detergent
2
Allow cells to dry completely before reinstalling (wet cells can arc and trip faults)
3
Check the EAC power supply for proper output voltage
4
Inspect ionizer wires for breaks and replace if damaged
5
Verify the interlock switch engages fully when the EAC is slid into position
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
EAC replacement cell
EAC power supply
Ionizer wire
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