Performed diagnostic on 17 year old comfort maker system. Homeowner states that heat is not working. It sounds like it is running but it never heats up and no hot air is coming out of the vents. Cycled system for heat. System initiated proper heating sequence, but gas valve never engaged and system goes into lockout mode afterwards. Gas valve is receiving proper voltage and command, but is not engaging. Gas valve is bad and needs to be replaced. Also performed a full safety evaluation on system. Pulled off Knox rods and checked heat exchanger to ensure safe operation. Found heat exchanger is compromised and each chamber is cracked. This is a safety concern due to combustion air possibly mixing in with ambient air. Went over findings and recommendations with homeowner. Did advise that If we were to repair the gas valve and not change the heat exchanger, gas or breaker to furnace would have to be left off and it would be up to homeowners judgement to turn it back on and operate the syste