Pkg clean and tune. Furnace. Cycled system. Checked heat exchanger. Found exterior of heat exchanger looked ok. Checked interior of heat exchanger. Found several areas of deep etching and large rust spots. This indicates weak spots in heat exchanger. Checked and cleaned burners. Checked electrical systems. Checked for CO outside of flame shield. Checked for CO in airstream. No CO found in airstream. This system is very old and showing signs of high energy use. Parts for this unit will be very difficult to find and very expensive. Due to age, condition, and cost to repair, second opinion is recommended. AC. cycled system. Checked electrical compartment. Capacitor tested lower but ok. Contactor is ok. Used coil cleaner on condenser coil. Hooked up gauges and checked pressures and temps. Indoor coil was at 41, subcooling was 10, superheat was 15, indoor temp split was 18. System is worn, but operating normal at this time. Job complete.