E106 Twin communication fault

What the control is telling you

Twinned furnaces have lost their twinning link. Usually one furnace has lost power, or the 24 VAC supplies of the two furnaces are out of phase.

Applies to: ML180UHEK (R-454B era) single-stage 80% furnaces with integrated control 107792-XX (ML180UH045/070/090/110/135, incl. low-NOx X models)

Likely causes

• Power off at one of the two twinned furnaces
• 24 VAC secondaries of the two furnaces not in phase
• Bad or loose wires on C, O and TWIN between the two controls

Test and repair steps

1. Confirm both twinned furnaces have 120 V power and their doors/interlocks are closed.
2. Check the twinning wires C, O and TWIN terminals must be connected between the two controls; all thermostat wires go to one control only.
3. Measure 24 VAC across the R terminals of both furnaces with the transformers in phase; a reading near 48 V means they are out of phase - reverse the primary or secondary leads on one transformer.
4. On the furnace without thermostat wires, the W call must come from a field-supplied 24 VAC isolation relay (coil W to common, contacts R to W) so its rollout switch cannot be bypassed by the other furnace.
5. Restore power to both units together and confirm the code clears.

Settings for these units

• Filter sizes by cabinet width · Lennox ML180UHE