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 communication bus. This is most commonly the outdoor unit (air conditioner or heat pump) being slow to respond to the indoor unit's polling requests.
The SLP98UHV communicates with other system components over a proprietary bus, periodically polling each device. When a device does not respond within the expected timeframe, the control board logs E120. This can happen after a power outage when devices initialize at different speeds, during extremely cold weather when outdoor unit electronics take longer to start up, or if there is an intermittent wiring connection.
In many cases, simply cycling power to the entire system resolves the issue. Turn off both the indoor and outdoor unit breakers, wait 30 seconds, then restore power to the outdoor unit first, followed by the indoor unit a minute later. If E120 keeps returning, a technician should check the communication wiring for loose connections or damage.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Outdoor unit slow to respond to communication polling from indoor unit | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- E120 returns after power cycling the system
- The outdoor unit is not responding to any commands
- The communication wiring needs inspection for damage or loose connections
- E120 appears alongside other communication codes like E105 or E124
✓ Verified against manufacturer service manual — March 2026