From 1999 onwards these cars had two lambda sensors, one before the cat and one after. The one before the cat is used to adjust fuelling. The one after it is used to assess whether the cat is working. And that's why it's become law, I reckon, to stop people running de-cat.
Now, whether that's the cause of your high CO, I don't know. I'm not entirely sure how a cat works, to be entirely honest. I'm guessing it's no longer performing it's chemical function and not scrubbing the air, or something? And the fact it's failing at this task is being detected perhaps by your second lambda sensor and putting the light on. But this is speculation, I'll admit.
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