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Q: failed nox ppm text on 1993 Toyota 4Runner

had cat converter replace last year. but it failed again this year what could be the problem.
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So many questions have to be answered to get to your problem.
Converters don't fail often. Usually something upstream of the coverter is what causes it to fail. Wether that is being caused by faulty O2 sensors (false readings causing a rich/lean mixture problem), raw/unburnt fuel getting into exhaust stream, worn/fouled spark plugs, excessive crankcase blow-by, oil/coolant being burned in the cumbustion chamber, or a myriad of other potential problems that might be damaging the converter or clogging it up to the point of failure.
Your car needs a comprehensive inspection of emission components and engine management to look for problems.
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