Lennox furnace E106: Twin communication fault
E106 appears only on twinned installations — two furnaces wired to operate as one system. It means the two controls are not communicating, and Lennox names two specific reasons.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, installed as a twinned pair, per installation instructions 508565-01.
What it means
Twinning is two furnaces sharing a duct system and controlled together, so their blowers start and stop in step. It is common in large homes and in light commercial work, and rare in an ordinary house. The two controls exchange signals to stay synchronised, and E106 means that exchange failed. Lennox gives two causes rather than a general troubleshooting list, which is unusually specific: either power was removed from one furnace and not the other, or the 24 volt supplies to the two units are not in phase. The first is the common one and is often a person's doing — a breaker switched off, a disconnect left open, a service visit that powered down one unit.
Most common causes
- Power removed from one furnace and not the other, including a tripped breaker or an open disconnect.
- The 24 volt supplies to the two furnaces not being in phase with each other.
- The twinning wire between the two controls broken, disconnected or damaged.
- One of the two control boards having failed.
- A transformer replaced on one unit and wired with opposite phase.
How to diagnose it
- Confirm both furnaces have power. Check both breakers and both disconnect switches — this is the most common cause and it is homeowner-checkable.
- Note whether either unit was serviced recently.
- Note whether one side of the house is noticeably colder, which is what a half-working twinned system produces.
- Phasing is a measurement and belongs to a technician.
How to fix it
- Restore power to both units if one is switched off.
- If both have power, the 24 volt phasing and the twinning connection between the boards need checking, which is technician work.
- Getting the phase right matters — two furnaces fighting each other in one duct system is worse than one running alone.
Notes
If the furnace is a single unit rather than a twinned pair, this code should not appear at all, and its presence would point at a control or configuration problem rather than a twinning fault.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.