Error Code E 120
Low

Lennox EL296UHV Error Code E 120: Unresponsive Device

TL;DR
Your Lennox EL296UHV is showing a communication error because an outdoor unit was slow to respond. A simple power cycle usually resolves this.
Disclaimer
This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code E 120 Mean?

Error code E120 on the Lennox EL296UHV indicates an unresponsive device in the communicating system. This is a communication-only issue — it does not indicate a mechanical failure of the furnace itself. The most common cause is the outdoor unit (air conditioner or heat pump) being slow to respond when the indoor control board polls it.

The EL296UHV uses a communicating bus to exchange data with other system components like thermostats and outdoor units. When a device fails to respond to a polling request within the expected timeframe, the control board logs E120. This can happen after a brief power interruption, during extremely cold weather when the outdoor unit takes longer to initialize, or if there is a loose wiring connection in the communication circuit.

The error clears automatically once the unresponsive device responds to any inquiry from the control board. In most cases, simply recycling power to the system resolves the issue.

Common Causes

Cause Likelihood DIY?
Outdoor unit slow to respond to polling Most common ✓ DIY fix →

How to Fix It: Power Cycle the System

⚠ Safety First
Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

Steps

  1. Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Also locate the outdoor unit's circuit breaker and flip it to OFF. Turn the gas shutoff valve on the furnace gas line to the OFF position (handle perpendicular to the pipe). Wait at least 30 seconds for the control boards to fully discharge.
  2. Check visible wiring connections With the power off, visually inspect the low-voltage communication wires where they connect to the furnace control board. Look for any obviously loose or disconnected wires. Do not pull or tug on wires — just check that connectors are seated.
  3. Restore power to the outdoor unit first Turn the outdoor unit's circuit breaker back to ON and allow it 1-2 minutes to initialize before restoring power to the indoor unit.
  4. Restore power and gas to the furnace Turn the gas supply valve to ON (handle parallel to the pipe) and flip the furnace circuit breaker to ON. The control board will begin polling connected devices.
  5. Monitor the system Set your thermostat to call for heat and observe the furnace through a heating cycle. The E120 error should not reappear once communication is re-established.
How to Verify
The furnace should complete a normal heating cycle without the E120 code returning. If you have access to the seven-segment LED display on the control board, it should show normal status codes during operation rather than E120.

When to Call a Professional

Contact a licensed HVAC technician if:

Sources

  1. Lennox EL296UHV Unit Information (Corp. 1125-L5)

✓ Verified against manufacturer service manual — March 2026