Performed clean and check on gas furnace. Conducted combustion analysis, measured temperature rise, tested all electrical components, inspected gas supply, drains, inducer, burner and blower assemblies, cleaned flame sensor, cleaned condensate trap and changed filter. Found high CO readings on combustion analysis (1800ppm). Inspected flu pipe and fresh air pipe. Opened secondary fresh air port and CO readings dropped to 167ppm. Tested for CO in the house and ductwork, found no traces. Discussed issue with Nate. Notified customer of likely secondary heat exchanger beginning to fail. Discussed repair vs replace options but left furnace running as it is since CO levels are no longer in hazardous range. System is otherwise operating normally at this time.