Completed 1 system furnace repair and preventative maintenance check. Changed the filter. Wiped out the furnace cabinet and cleaned the burners. Cleared the barometric tubing and cleaned the pressure switch port and the inducer port. Tested the combustion and found proper oxygen and CO levels. Tested the mechanical and electrical components and found a weak flame sensor. The system has no inline water protection to protect the furnace from water damage. Recommended adding it to the system. Replaced the flame sensor and installed secondary water safety. The system is heating at this time.