I was called to a home to inspect a gas furnace. Upon arrival, I noticed that the flame sensor was dirty and needed to be cleaned. I used a brush to clean the sensor, and after that, the furnace started working properly. I also noticed that the gas shut-off indicators were showing codes 11 and 12, which indicated that the gas was shutting off. I reset the gas shut-off and tested the furnace to ensure it was working properly.