I know of no way to fix a heater, and I would not mess with trying to do that. It is too dangerous for the health of the fish.
I was just about to make a venting post about the quality of aquarium heaters. I've noticed a major decline in quality over the past 20 years. I recently bought two new heaters rated for my tank sizes. No, they weren't the most expensive type, but they both have good reviews and aren't bottom of the barrel, either. One turned out to be completely inadequate for the size of the tank despite being rated for that tank size. It won't even heat the tank up to a standard temperature. The other has wild fluctuations in temperature--either goes off when it shouldn't or stays on when it shouldn't. I am thoroughly disgusted with both. The next time I buy a heater, I am going to do a lot of research to make sure I get one with superlative reviews and a warranty.