My co% is almost double. But I filled half a tank and used cleaning fuel additive, and took test only after 1/4 tank. Do you think testing with the additive still in caused high co readings??here are readingsHC 1.7 limit 4.0 passCO 79.9 limit 40.0 failNOx 0.06 limit 6.0 pass‘86 mazda b2000
Tank added additives do little to nothing more than cost you money. Sometimes they help, but more often not. I really doubt that will cause your troubles. It is hard to give great advice without having a diagnostic, but I would suspect a tired oxygen sensor.the one before the catalytic converter. When they get tired they get slow and tend to stay rich, causing your high CO readings. The low HC indicates no misfire, and that's a really good thing.
HC (hydrocarbon) limits indicated a proper fuel/air mixture. NOx (Nitrogen dioxides) are the result of the catalytic converter doing its job and breaking down heavier nitrogen gases. CO ( carbon monoxide) is basically raw fuel or the product of incomplete combustion of fuel. Most CO is produced when air-to-fuel ratios are too low in the engine during vehicle starting, when cars are not tuned properly, and at higher altitudes, where thin air reduces the amount of oxygen available for combustion. It may be as easy as a tune up or a set of plugs. The additive most likely did NOT contribute to the high readings. Remember too that a half full tank of fuel has a half full tank of fumes or raw fuel vapors. GM Tech since 1990 and cert'd Ontario Drive Clean Tech.
I doubt it is the additive - I bet your O2 sensor is the original and it was useless ten years ago. O2 sensors degrade after about 100K miles (Bosch says after 60K miles, but they like to sell them) and that is the most important factor in CO emissions. If that doesn't do it you are probably looking at a replacement catalytic converter.
Local tyre dealer will know what's on offer so off ya go there and ask.
