Ultimate Heating Tune-Up Gas Heat: serviced and inspected gas heating system checking air flow, temperature rise, electrical, gas pressure, safeties, controls and limits, heat exchanger and venting. cleaned and checked system components to manufacturer specifications and performed combustion inspection. We missed the spring tuneup. Went ahead and did the fall maintenance under the previous contract. Basement unit had carbon monoxide above 575 ppm. Pulled burners, clean burners adjusted gas valve brought carbon monoxide down to 40 ppm. Looking into the heat exchanger all signs of rust which could lead to failure of heat exchanger.