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.
How a twinned pair is actually wired
The twinning section of 508565-01 explains what E106 is guarding. Two ML193UHEK furnaces are twinned by connecting the “C”, “O” and “Twin” terminals between the two controls; the 24-volt secondaries of the two units must be in phase; and all thermostat wiring lands on one control only. The furnace without thermostat connections gets its heat call through an external 24-volt isolation relay (Lennox kit 56L68) — the manual is explicit that this relay exists to prevent that unit's rollout switch from being bypassed by its twin. When the pair is healthy, blower speeds are synchronized with cooling speed taking priority over heating and fan. That wiring picture is why E106's two named causes are what they are: one unit losing power breaks the conversation, and out-of-phase 24-volt supplies corrupt it. It is also why nobody should improvise a fix — the safety interlocks of two gas furnaces are deliberately interlaced here.
What the fix costs
If the cause is the common one — one furnace powered down — the cost is zero and the fix is a breaker or disconnect switch. Past that, expect a diagnostic visit at $75–150 for a technician to verify 24-volt phasing and the twin connection between boards; a phasing fix is wiring labor, not parts. The expensive branch is a genuinely failed control on either unit, which puts you at $400–900 installed for a board — and on a twinned system the technician should establish which of the two boards stopped talking before either is replaced.
DIY or call a pro
The first check is legitimately yours: confirm both furnaces have power at both breakers and both disconnect switches, since a half-dead pair is the most common cause of this code and takes two minutes to rule out. Everything after that is professional work for a twinning-specific reason — verifying 24-volt phase and the isolation-relay wiring means live measurement across two interconnected gas furnaces whose rollout protection depends on that wiring being exactly right. Miswiring a twin doesn't just fail to fix E106; it can silently defeat a safety switch on one unit. If both units have power and the code stands, stop there and book the visit.
The same fault on other brands
Twinning faults are rare enough that only a few control families give them a code, and the site documents them together: the shared Goodman/York board reports a twinning error as rapid red flashing (York's version, rapid red, adds incorrect 24-volt phasing to the description — the same phasing rule Lennox enforces here), and Rheem dedicates 5 blinks to a twinning circuit failure. Same paired-furnace fault class, same first question: do both units have power?
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.