E421
Lennox
Pressure Switch / Inducer Draft Fault (communicating report)
Thermostat shows E421; furnace may start inducer but fail to light burners.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked PVC intake or exhaust vent, Condensate trap or drain line clogged, Faulty pressure switch, Weak or failing Lennox inducer motor
How to Fix / Troubleshooting
Safety: Turn off power and gas before working on venting or internal components.
- 1. Inspect vent pipes: Check the Lennox furnace PVC intake and exhaust terminations outside for leaves, snow, bird nests, or debris. Clear any obstructions.
- 2. Check condensate drain: For condensing Lennox models, inspect the condensate trap and hoses for sludge or kinks. Flush the trap with warm water and reassemble, ensuring proper slope to the drain.
- 3. Inspect pressure switch tubing: Ensure the rubber tubing from the inducer to the pressure switch is not cracked, kinked, or filled with water. Replace or clear as needed.
- 4. Reset and test: Restore power and call for heat. Listen for the inducer and see if the furnace proceeds to ignition.
If E421 persists, the pressure switch or inducer assembly may require professional testing and replacement.
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Repair Difficulty
Hard
4/5
Required Part
Pressure Switch
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