My heater displays e3 code and beeps. ?

Garrison Oscillating ceramic tower Heater. Model: 043-5931-2
My heater keeps displaying e3 code and beeping at the same time. It does this frequently. I put it on at night to stay warm and it doesn't let me sleep. It used to happen rarely before, but now it has become very frequent. For example, once in 30 min and sometimes 2-3 times in 30 minutes or even more at other times. Sometimes it doesn't beep for an hour. It's very irregular, but consistent enough to be very irritating at night. I can't figure out what is causing it. It didn't used to do this when I first bought it. It's about 1-1.5 years old.
Also suggest better heaters within $20-50 range for a 15 by 10 feet room for sleeping at night. I like the room to be 25 degree celcius.