Engine Light comes on, especially in warm weather

I went to Nate Wade Subaru in Salt Lake City to have them run a diagnostic check to see why its happening. I paid them money for the diagnostic and I told them if its a knock sensor of O2 sensors I would do the work myself. The service guy reluctantly told me that it was either the knock sensor and the O2 sensors. So I went ahead and did the O2 sensors but the engine light still comes on. Is this common?