Was at a call the other day for CO detector alarming(turned out to be low battery alarm), anyway while there did co test on furnace & hwh. Furnace was ok but found high co levels in undiluted flue gas at hwh diverter(550ppm)Shut unit off & cleaned burner & pilot and pulled baffle out & cleaned as well also cleaned inside of flue travel as best I could. Re-assembled and fired up unit, still had 400ppm in undiluted sample. Heater was rated for 36000btu and I clocked it at 36,300btu so over-firing not an issue.I shut the gas off to the hwh and tagged it advising customer to have service co. give unit a thorough cleaning. Customer called the office today stating service co came and worked on unit but could not get levels down to under 200ppm. My question is are there any service bulletins addressing this issue with these hot water heaters that I am not aware of? I did notice that this unit has a strange flue baffle in that it is flat with 1/2 moon shaped plates sticking out horizontally ,which to me would restict the flue products too much, in turn may be recirculating into the combustion process causing the high co levels?Any ideas?