Performed package clean and tune; inspected the condenser electrical compartment and tested components system keeps short cycling, i bypassed Tstat problem persisted, replaced contactor still same issue, sniffed the evaporator coil for possible leaks did find quite a few leaks, No cool until refrigerant is added, removed and cleaned the burners along with flame sensor, tested igniter which did test weak, inspected the heat exchanger did find a substantial amount of rust, cycled the furnace on and checked gas pressures, as well as performed a combustion test CO is over 90ppm with over 200 excess air this system is not safe to operate at this time. I recommend looking into options. Job complete