smog detector on 2001 Subaru Forester

85,000 miles
check engine light comes on, goes off after several days, comes on again
mechanic said it was a smog detector?
please locate this for me and offer insight into replacement

Asked by for the 2001 Subaru Forester
There is no "smog detector". Here is background on the check engine light that will get you started on the right path to getting the problem fixed: