No heat basement. Found system turned off. Upon arriving turned on. System appeared to operate but is very loud. Upon inspection found cracked heat exchanger, high amp draw on inducer motor, weak run capacitor, dirty blower wheel, high limit was tripping.. found system to be leaking carbon monoxide into basement. Upon trying to price repairs was notified parts no longer available for system as it is 25 years old and not in production anymore. Furnace needs to be replaced. Informed customer. Customer wants quote to replace furnace. Informed customer of crack in heat exchanger and that it was leaking carbon monoxide. Advised due to safety concerns not to operate the heat. Left thermostat in off position.