Lennox communicating thermostat shows E420 high line voltage or power quality error; system may lock out or run intermittently.

Safety: High voltage issues are hazardous. If you are not experienced with electrical work, contact an electrician or HVAC technician.

• 1. Verify supply voltage (professional): Using a multimeter, measure line voltage at the outdoor disconnect and indoor unit. Compare to Lennox nameplate rating (usually 208/230V). Values significantly above 253V can trigger faults.
• 2. Inspect disconnect and lugs: With power off, tighten all lugs in the disconnect and at the unit's line voltage terminals. Look for signs of overheating or arcing.
• 3. Check grounding: Ensure the ground wire is securely connected to the unit and the electrical panel. Poor grounding can cause erratic control behavior.
• 4. Consider surge protection: If you experience frequent power fluctuations, have a whole-house or dedicated HVAC surge protector installed.

Note: Persistent E420 errors should be evaluated by a licensed electrician to prevent damage to the Lennox control boards and compressor.