If you are not sure how to properly test power( or use a multimeter) then you need to demand that the original installer come and fix your problem. Call them every hour on the hour until they get their worthless selves out to fix your problem. Fact is , most likely they are installing furnaces, and don't have a clue as to how they work. Therefore they won't come work on it because they don't know how to trouble shoot it.
If you have a friend that knows how to read a electrical schematic, and is handy with a multimeter, then have them sit at your computer, and we will help them solve your problem.