Lennox EL296UHV Error Code E 120: Unresponsive Device
What Does Code E 120 Mean?
Error code E 120 on the Lennox EL296UHV means the SureLight integrated control asked a device on the communicating bus to respond and did not get an answer in the expected time. It is a communication-only event. Nothing inside the furnace has failed mechanically, no safety device has tripped, and there is no airflow or gas problem behind this code — so a dirty filter is not the issue here.
Because the EL296UHV is an icomfort-enabled unit, the indoor control, the communicating thermostat, and the outdoor unit all talk to each other over a shared data bus rather than simple on/off wires. The control continuously polls each device. If one device is slow to reply — most often the outdoor air conditioner or heat pump — the control records E 120 for that missed reply. This is why the code names an unresponsive device rather than a specific broken part.
Slow responses commonly follow a brief power blip, a very cold morning when the outdoor unit takes longer to wake up, or a loose connector on the two-wire communication link. Lennox designed the code to self-heal: E 120 clears automatically as soon as the quiet device answers any later inquiry from the control. That is why a full power cycle, which forces every device to re-introduce itself on the bus, is the standard fix.
E 120 belongs to the same communication family as E105 and E124 on this board. If those codes appear alongside E 120, the problem is more likely the bus wiring or a failing device than a one-time slow reply, and the pattern points a technician toward the shared communication link rather than any single component.
What You'll Notice
- The seven-segment LED on the SureLight control shows E then 120
- The furnace may still heat normally, or heating and cooling calls are intermittent
- The communicating thermostat briefly loses data from the outdoor unit or shows a connection message
- The code often appears after a power flicker, a storm, or on a very cold morning
- The code disappears on its own once the outdoor unit answers the control again
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Outdoor unit slow to respond to polling | Most common | ✓ DIY fix → |
How to Fix It: Power Cycle the Communicating System
What You'll Need
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Flip the furnace circuit breaker (or the service switch on the furnace) to OFF, and also flip the outdoor unit's breaker to OFF. Turn the furnace gas shutoff valve to the OFF position, with the handle perpendicular to the pipe. Shutting off the gas is a standard safety precaution any time you work at the furnace. If you smell gas, leave immediately and call your gas company.
- Wait about 30 seconds Leave everything off for roughly 30 seconds so the SureLight control and the outdoor board fully discharge and drop off the communicating bus. This is what forces every device to re-introduce itself when power returns.
- Restore power to the outdoor unit first Turn the outdoor unit's breaker back ON and give it a minute or two to initialize before you power the furnace. Letting the outdoor unit come up first means it is ready to answer the control's first poll.
- Restore gas and power to the furnace Turn the gas shutoff valve back to ON, with the handle parallel to the pipe, then flip the furnace breaker or service switch back ON. The SureLight control will boot and begin polling the devices on the bus.
- Let the system re-establish communication and observe Set the thermostat to call for heat and watch the furnace through a cycle. Give the system a couple of minutes to finish its handshake. E 120 should not return once every device has answered the control at least once.
When to Call a Professional
Contact a licensed HVAC technician if:
- E 120 returns after a full power cycle of both the furnace and the outdoor unit
- E105 or E124 appear together with E 120, pointing to a bus wiring problem
- The outdoor unit does not power up at all after the breaker is restored
- The communication wiring looks damaged, chewed, frayed, or disconnected
- The thermostat repeatedly drops its connection to the outdoor unit throughout the day
Frequently Asked Questions
Is E 120 dangerous or will it damage my furnace?
No. E 120 is a communication-only code, not a safety or overheating fault. It reports that a device was slow to answer the control, and it usually clears on its own or after a power cycle.
Do I need to replace my air filter for E 120?
No. The filter has nothing to do with this code. E 120 is about the communicating bus, not airflow, so replacing the filter will not affect it.
Why does E 120 show up on cold mornings?
On very cold mornings the outdoor unit can take longer to wake up and answer the control's poll, which the SureLight control logs as an unresponsive device. It typically clears once the outdoor unit responds.
The code cleared by itself — should I still be concerned?
A single self-clearing E 120 after a power blip or cold start is usually nothing to worry about. If it keeps coming back, or if E105 or E124 appear with it, the communication wiring should be checked.
Sources
✓ Verified against manufacturer service manual — March 2026