Check Engine Subaru Legacy 97

Hello, I have an Automatic Legacy Subaru of the 97, which after walking about 20 or 30 minutes, lights the light of the check Engine, lasts a while on and turns off, the problem is that when the scanner goes out the scanner does not detect where the fault is from, I would like to help me and indicate where the fault could be, my mechanic tells me that it could be the oxygen sensor, but there is still no certainty.

Compa, what you comment is difficult. Any scanner has access to the Frezze Frame that leaves frozen data on the last DTC (revolutions, temperature, sensor O2, air volume, speed, etc.) especially because it is automatic. It should be taken to another scanner .....

Thank you friend for your answer, take it to another scanner and throw the detonation sensor and oxygen, change them and subject matter, thank you !!