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Q: does replacing one catalytic converter sometimes cause the other to go bad? on 2005 Ford Expedition

had check engine light code saying i had a bad catalytic converter on the left side, had it replaced, 2 days later the light came back on with the same code, took it back in, and was told now the right side is bad, that this is common when you replace 1, it can make the other go bad. Makes no sense to me.
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If on different sides I doubt that one caused the other to go bad. Typically cats go bad because of contaminants getting into catalyst and causing meltdown, raw fuel causing same, usually from failing oxygen sensors and worn out tune related components or failing ignition system or fuel flow regulation problems. If they were the same age then one could go bad after the other because of listed reasons effecting both before the first went bad.
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one cannot make the other one go bad. verify the code with the repair shop as they are different for each side. p0420 is for bank one (right side) and p0430 is for bank two (left side). these are cat. conv. efficiency codes. this does not mean the converters are bad, only that the o2 sensor signals are too close to a mirror image of themselves. having said that, Ford converter failure is fairly common for many reasons(as mentioned by raiderron) FYI those converters are covered under your emission warranty by the manufacturer 8 years/80,000 miles
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