E312
Lennox
Outdoor Unit / Furnace Communication Error (for dual-fuel or integrated systems)
Thermostat shows E312; furnace may run but with limited control or may not respond to thermostat changes.
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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
Disconnected outdoor unit communication wiring, Faulty Lennox outdoor unit control board, Misconfigured dual-fuel settings, Damaged communication cable between furnace and outdoor unit
How to Fix / Troubleshooting
Safety: Turn off power to both furnace and outdoor unit before working on wiring.
- 1. Inspect communication wiring: Locate the communication cable between the Lennox furnace and outdoor unit. Ensure all terminals are tight and correctly matched (ABCD or designated terminals).
- 2. Check for damage: Look for UV damage, rodent chewing, or splices exposed to moisture. Replace damaged sections with proper outdoor-rated cable.
- 3. Verify configuration: In the thermostat installer menu, confirm that the system is set up correctly for heat pump or dual-fuel operation and that the outdoor unit type matches the installed Lennox model.
- 4. Power cycle: Restore power to both units and allow several minutes for communication to re-establish.
If E312 persists, a technician should test the outdoor unit control board and communication bus.
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Repair Difficulty
Hard
4/5
Required Part
Lennox Outdoor Unit Control Board
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