Diagnosis
The E1 code is an overheat protection alert. The internal thermal sensor has tripped because the heater core got too hot.
How to Fix It
Unplug the heater immediately. Let it cool down completely for at least 30 minutes. Check the air intake (back) and outlet (front) for any obstructions like dust, lint, or fabric. Clear them. Move the heater to a location with at least 3 feet of clear space on all sides. Plug it back in.
If That Didn't Work
The problem could be a faulty or dust-clogged temperature sensor. You need to access the internal components. Unplug it, remove the rear grille, and use compressed air to blow out all dust from the heating element area and the sensor near it.
Heads Up
Never run this heater on high power setting in a small, enclosed space. It will cycle the overheat protection constantly. Use a lower wattage setting or improve room ventilation.