What this code means
The official model-family manual identifies E401 as compressor ran more than 18 hours in air-conditioning mode.
Safe checks
- Confirm the thermostat is on, set correctly, and calling for heating or cooling.
- Check the air filter and visible return-air path for obvious airflow restrictions.
- Record the displayed code or flash sequence before scheduling service.
First step
Record the code, avoid repeated reset attempts, and schedule GreenFlow service.
Frequently asked questions
What does E401 mean on a Lennox SLP99DF070XV36B*?
The official model-family manual identifies E401 as compressor ran more than 18 hours in air-conditioning mode.
What should I safely check before scheduling service?
Use only the visible, non-invasive checks listed on this page. Do not open equipment panels.
When should GreenFlow handle this code?
Call GreenFlow if the code repeats, the equipment is locked out, heating or cooling is not restored, or the condition involves gas, ignition, pressure, wiring, a motor, or a control board.