Lennox SLP98UHV Error Code E 105: Device Communication Problem
What Does Code E 105 Mean?
Error code E105 on the Lennox SLP98UHV means the SureLight variable-capacity control sees no other devices on the BUS communication system. On this communicating furnace, the control board, iComfort thermostat, and outdoor unit exchange data over a shared bus; E105 says the board is talking but nothing is answering.
The most common cause is a loose, broken, or mis-wired communication connection. The bus runs as a chain between the furnace, thermostat, and outdoor unit, so a single bad connection, a reversed pair, a rodent-chewed wire, or a corroded terminal anywhere in that chain can silence the whole system. E105 is the broadest form of bus communication loss on this board: where soft disable means the thermostat finds a device on the bus it does not recognize, and E120 means a device is present but slow to answer polling, E105 means the control detects no peers on the bus at all — a complete absence of response rather than a single misbehaving device.
A less common cause is electrical noise. A nearby high-voltage source such as a welder, large motor, or electrical panel can inject interference that corrupts the low-voltage communication signal. The code clears once communication is re-established, so a technician should inspect every bus connection end to end and check for sources of electrical interference near the system.
What You'll Notice
- E 105 shown on the furnace control's 7-segment display
- The whole system is unresponsive — no heating or cooling even though power is on
- The iComfort thermostat may show a blank screen or a 'no communication' / loss-of-communication message
- The problem started after recent service, a wiring change, or a storm
- Other communicating devices in the system are also reporting communication faults
Common Causes
How This Is Diagnosed
A technician isolates E105 by tracing the communicating bus end to end. Each device is checked to confirm the communication conductors are landed on the correct terminals with the right polarity and are tight, since one loose or reversed connection breaks the entire bus. Damaged, chewed, or corroded sections of the communication cable are identified and repaired.
If the wiring checks out, the technician looks for a source of electrical noise near the system — a welder, large motor, or run of high-voltage wiring beside the communication cable — that could be corrupting the signal. Power is cycled and the bus is re-checked to confirm the devices reappear and communicate.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The communication wiring between system components needs professional inspection
- E105 appeared after recent service work or a storm
- The thermostat and furnace are both unresponsive
- A welder, large motor, or other electrical-noise source operates near the furnace or its wiring
Frequently Asked Questions
Why is my whole system dead with E105?
On a communicating SLP98UHV, the furnace needs the data bus to receive heating and cooling commands. When the board detects no devices on the bus, it has no instructions to act on, so the system stays idle until communication is restored.
It started right after someone worked on the furnace — is that related?
Very likely. A communication wire that was loosened, reversed, or left off during service is one of the most common triggers for E105. The connections disturbed during that work should be the first thing checked.
Can electrical interference really cause this?
Yes, though it is less common than a wiring fault. Strong electrical noise from equipment like a welder or a large motor near the communication cable can corrupt the low-voltage bus signal enough to break communication.
✓ Verified against manufacturer service manual — March 2026