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P0420 Catalyst efficiency _APPEARS_ to be below threshold for more than
three cycles. If either of the O2 sensors goes bad, the computer may
come to the conclusion that the cat is bad.

Any 'issues' with the engine since the dealer (recently) touched it? Do
you use injector cleaner seasonally, has the engine ever stalled,
especially during warm-up, do you ever get idle speed going up and down ???

If it really _is_ the cat, you need to determine what killed it, or
you'll soon be putting another in there. I'd get a generic cat but only
after I made sure the O2 sensor(s) are OK.

'Curly'

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Here's an important one for cat health: Is the coolant reservoir full to
the =MAX= mark, per the owner's manual? The dealer knows they have to
fill it up any time the cooling system is apart.

If there's air in the cooling system the computer can set the mixture /
timing all wrong which will shoot your mileage all to pot, and wreck the
cat. Has you mileage been exemplary since the timing belt was done?

'Curly'