failed emission test, too much polution Options

[celestrial_rider]

so my '94 celica gt failed its emission test again (2nd time) this past week. it gave me the same reading that im making too much CO. i changed the oil, oil filter, air filter, sparkplugs, even poured in a bottle of seafoam into a full gas tank..and it still fail. i was given a few options b4 my next emission..either i replace the knock senser, or replace the O2 senser (a buddy recommended either, or both). just curious if u guys can piont out anything else that might b causing this high level of polution..just anything. thanks.