Returne to give second opinion on furnace combustion issues. I found that gas pressure was set at 4.5 inwc. This is far too high. Adjusted gas pressure to manufacture specifications and CO levels in flue pipe dropped. Furnace is still operating between 280-320ppm in flue pipe. This is still very high level for furnace. After checking all combustion components, there was no other issue. We Would have to start replacing part by part to determine issue. Due to the age of the system that is far from recommended. Certified that furnace is safe to operate at this time per Code. Water heater venting needed to be re pitched to avoid red tag of water heater. Venting now correct.