engine management warning light

[Davy60]

I recently got a new cat fitted but since then my engine management warning light comes on for a few day's and them goes out for another few days. I got a mechanic to have a look at the diagnosis and he told me that one off the sensors needed changed, has anyone had this problem before I would like your opinions,by the way I have a 2003 subaru forester. 

[Phillip1993]

O2 sensor is on its way out mine did the same

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[Davy60]

Cheers Philip at least I now know what the problem is.