Today we came out for only furnace not heating 2000 trane 120k with 2012 rheem 4 ton. Cycled equipment from thermostat found igniter working but gas valve solenoid not opening. Checked for 24 volts and found proper voltage at valve gas valve has failed. Performed safety inspection with digital camera on furnace. Inspected heat exchanger found cracks at 5 and 7 o clock through high limit. Tagged furnace unsafe to operate in heating mode due to potential carbon monoxide fumes. Gas piping is not up to current code flexible gas line tied into gas valve must be hard pipe coming out of furnace cabinet. Life spans on a furnace is 14-16 years on an average. Left band aid repair option and scheduled install supervisor to give options on updating equipment.