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Message: <blockquote data-quote="JorgeS" data-source="post: 26480" data-attributes="member: 12094">My 1998 GMT400 threw the "catalytic converter below threshold" code intermittently for a long time; it would light up, then some time later it would go out, then back, and so on. I checked it with a scanner, and it would store the code even after the light went out. I never replaced the converter. My understanding of that code is as follows:Converters have the ability to store a small amount of oxygen, to compensate for a short-term rich mixture.Once everything is up to temperature, and stable, the engine runs slightly lean for a short period, "storing" O2 in the converter. Then the engine runs rich for another short period, using that "stored" O2. There is a time lag between the front and rear O2 sensors detecting the rich condition. This is because, initially, the excess fuel is being oxidized by the "stored" O2. If the lag is too short, it means that the catalytic converter has lost some of its "storage" capacity, is below the threshold, and the code is set.I don't believe you're putting out high emissions on a regular basis; it might be for very short periods during those short rich periods, but that would be all.Unless you have to pass an emissions test, and having the light on will fail you, I'd consider doing nothing, and keeping an eye for other faults that might indicate a more serious converter issue.</blockquote>