Toyota Repair: toyota corrolla 2001 check engine light., air fuel ratio, catalyst switches

Question
The cat is new, as is its oxy sensor and the muffler The check engine light came on after only 808 miles after the new cat was installed.The code po420 has to be for something else in the system. I have 116000 miles on the car. Could it be the spark plugs? It cannot be the cat or the 2 oxy sensors.
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The code is PO420 also the front oxy. sensor was replaced 20k earlier.
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808 mile ago I had everything behind the exhaust manifold  replaced. The check engine went out. 808 miles later it came back on. I cleaned the maf? or s? last year. The gas cap was on correctly. There was oil in the engine. Why did it come back on and what can I do about it?
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I need to know what the code is. Then I can help you diagnose the problem.
Do you know the code, if not have it scanned and let me know.

Mike
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p0420 is Cat Converter code.
The Sensors before and after the Cat read the same, so the Cat isnt working and it stores a code.
You need a Catalitic Converter.

Mike

Answer
DTC P0420 Catalyst System Efficiency Below Threshold
CIRCUIT DESCRIPTION
The ECM compares the waveform of the heated oxygen sensor located before the catalyst with the waveform
of the heated oxygen sensor located behind the catalyst to determine whether or not catalyst performance
has deteriorated.
Air-fuel ratio feedback compensation keeps the waveform of the heated oxygen sensor before the catalyst
repeatedly changing back and forth from rich to lean.
If the catalyst is functioning normally, the waveform of the heated oxygen sensor behind the catalyst switches
back and forth between rich and lean much more slowly than the waveform of the heated oxygen sensor
before the catalyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.

Thats straight from the repair manual.
I dont think I can make it any clearer myself so I cut and pasted it.

You could have bad o2 sensors, I saw that only once, but it did fix the car.
Is it a factory new Cat converter?

Mike