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With low CO & HC, but high NOx, you're engine is definitely running too lean.
Before assuming the cat's bad, I'd check the easy stuff first.
As mentioned before, check for vacuum leaks e.g., broken/cracked/disconnected vacuum lines.
Check O2 sensor, but a failure there usually results in overly rich mix, but it's easy to check (see procedure at Clarks Garage.)
Check proper operation of throttle position switch, also detailed at Clark's.
Check that someone didn't monkey with the air bypass screw. It's the one that's supposed to have a metal plug on top so that it's not played with. If the metal plug is in, it's probably ok. If not, adjust it to a more rich mixture. If you don't have a gas analyzer, as most of us don't, it'll be trial and error. Also detailed at Clarks.
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