Performed heating maintenance, checked air filter ok. Cleaned burners and flame sensor, inspected heat exchanger. Found rust running all the war down the seam. Blew out condensate trap and drain. Checked voltage, amperage and electrical connections, measured capacitor values. Found inducer running at full load amps and is noisy. Started gas furnace, tested limit and safety switches, measured fuel pressures. Performed combustion test. Found very high co pegged meter could not do combustion test. Recommend replacing furnace due to age, condition and needed repairs.