Why is my CHECK ENGINE LIGHT still on AFTER performing ALL necessary repair work

Check engine light diagnostic from Auto Zone, Advanced Auto, & Pep Boys, suggested BOTH 02 sensors (upper & lower) were defected. BOTH sensors replaced on two different occasions, in Jul & Oct by Al's Auto Clinic. WARNING LIGHT REMAINED ON. Probable causes, listed on the diagnostic printout, were subsequently done. WARNING LIGHT REMAINS ON. So.....