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Q: Code p0421

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I am getting a p0421 code but I just replaced the catalytic converter. I got the code erased after but it popped back on. My car seems to stumble/put a lot though. What else could it be?

My car has 148000 miles.
My car has an automatic transmission.

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Message: Invalid string length

RangeError: Invalid string length
    at t.s.value (/tmp/execjs20161130-87336-1y7zdzrjs:168:15384)
    at /tmp/execjs20161130-87336-1y7zdzrjs:168:16113
    at f (/tmp/execjs20161130-87336-1y7zdzrjs:61:12326)
    at Function.Uo [as map] (/tmp/execjs20161130-87336-1y7zdzrjs:62:8878)
    at t.s.value (/tmp/execjs20161130-87336-1y7zdzrjs:168:15665)
    at m._renderValidatedComponentWithoutOwnerOrContext (/tmp/execjs20161130-87336-1y7zdzrjs:47:19688)
    at m._renderValidatedComponent (/tmp/execjs20161130-87336-1y7zdzrjs:47:19776)
    at m.mountComponent (/tmp/execjs20161130-87336-1y7zdzrjs:47:16076)
    at Object.o.mountComponent (/tmp/execjs20161130-87336-1y7zdzrjs:47:5085)
    at m.mountComponent (/tmp/execjs20161130-87336-1y7zdzrjs:47:16172)

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