Lennox SLP98UHV Error Code E 120: Unresponsive Device
What Does Code E 120 Mean?
Error code E120 on the Lennox SLP98UHV indicates that the control board has detected an unresponsive device on the communicating data bus. Most often this is the outdoor unit (air conditioner or heat pump) being slow to answer the indoor unit's polling requests rather than a device that is truly absent.
The SLP98UHV, its iComfort communicating thermostat, and the outdoor unit exchange data over a shared bus, and the control periodically polls each device for a reply. When a device does not respond within the expected window, the board logs E120. This commonly happens after a power outage when devices power up at different speeds, in very cold weather when outdoor unit electronics take longer to initialize, or when a connection is intermittent. E120 sits in the middle of the bus family by severity: E105 means the board sees no devices on the bus at all, and E124 means the communicating thermostat signal has gone missing for more than three minutes, whereas E120 means the device is present on the bus but was simply too slow to reply.
Because the cause is often just a slow or momentary response, cycling power to the whole system frequently clears it. If E120 keeps returning, a technician should inspect the communication wiring between the indoor and outdoor units for loose or damaged connections.
What You'll Notice
- E 120 shown on the furnace control's 7-segment display or error code recall menu
- No heating or cooling until power to the system is cycled
- The outdoor unit does not respond to a cooling or heat-pump call
- The code first appeared after a power outage or during very cold weather
- The code clears after a power cycle but sometimes comes back
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Outdoor unit slow to respond to communication polling from indoor unit | Most common | ✗ Call a pro → |
How This Is Diagnosed
Because E120 is frequently a slow-response glitch, the first safe step a homeowner can take is to cycle power at the breaker: turn off both the indoor and outdoor unit breakers, wait about 30 seconds, restore the outdoor unit first, then the indoor unit a minute later. If the system comes back and stays online, the code was a transient polling delay.
If E120 returns, a technician traces the communicating bus between the indoor and outdoor units, checking that the conductors are landed on the correct terminals, tight, and free of corrosion or damage, since an intermittent connection lets a device answer sometimes but not others. The technician also confirms the outdoor control is powering up and responding to polling as expected.
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 code returns after cycling power at the breaker
- The outdoor unit is not responding to any commands
- The communication wiring between indoor and outdoor units needs inspection
- E120 appears alongside other communication codes such as E105 or E124
Frequently Asked Questions
Will cycling power fix E120?
Often, yes. Because E120 usually means a device was simply slow to respond to polling, restoring power to the outdoor unit first and then the indoor unit gives everything time to initialize and re-establish communication. If it comes right back, the wiring should be checked.
Why does E120 show up in cold weather?
In very cold conditions the outdoor unit's electronics can take longer to wake up and answer the indoor board's polling. If the reply arrives later than the control expects, it logs E120 even though the device is working.
How is E120 different from E105?
E105 means the board detects no devices on the bus at all, which is more severe. E120 means the device is present on the bus but responded too slowly, so it is usually the milder of the two communication issues.
✓ Verified against manufacturer service manual — March 2026