We were called out for a no heat on a Viessmann boiler, we found that the boiler is displaying a code F1 and a code B8. Both of these codes are related to high flu-gas temperatures. With approval, I opened up the boiler, and found that the heat exchanger was very dirty. This can lead to higher flu-gas temperatures, and other related issues. After discussing this matter with Sarah, I learned that the boiler had not been cleaned in quite some time, because of the lack of maintenance I advised that this would be an attempt to repair as sometimes the heat exchanger can be plugged beyond repair. I cleaned the heat exchanger in the boiler, the condensate trap, and the connecting tube. Everything cleaned up pretty well, when I started the boiler back up, it was still displaying a code for a failed flu gas temperature sensor. With approval I replaced the sensor and this resolved the error code. I started the boiler up performed a combustion test and the boiler seems to be running well at thi