Ice accumulation is normal due to condensation from the temperature change. The condensate then turns to ice when it is below freezing. Pilot lights can go out when the thermocouple fails or there is high wind coming down the flu or against a side of the building with a lot of air leaks
SOURCE: We have a Model # 3003622 Williams Gas heater. The
yes clear the ice and clean the pilot assembly sounds like the flame is being retarded by a dirty pilot orifice
SOURCE: pilot light wont stay on
Thank you for posting your question here on Fixya.com.
Has anyone replaced the pilot generator yet? A millivolt heater (one with a pilot light) requires about .75 volts from the generator to operate. As the generator ages it puts out less and less voltage and eventually gets to weak to even hold the pilot gas valve open. Replacing it with a new generator will usually fix the problem you describe.
If this answer does not fix your problem, please comment with additional details prior to rating the answer. Positive feedback is appreciated once your problem is solved!
John
SOURCE: pilot light wont stay on
There is a thermo couple that keeps the gas on. The pilot light bracket has a small screen where the gas comes through that can get clogged and cause the pilot light to burn away from the thermo couple which would cause it to think the flame has went out and shut the gas off. Or the thermo couple has went bad itself. If you try cleaning the screen use canned air and gently blow it through the orfice once you have taken it off. If you don't know what a thermo couple is or looks like here's a picture of a generic one that will give you an idea. Usually there's a bracket that holds this in place. 
115 views
Usually answered in minutes!
×