P0133 o2 sensor error code

[Kenco]

I had my OBD II come on so I took it in and had the O2 sensor replaced. A few days later same thing, returned to dealer and they replaced it free of charge. Took it home and a few days later same thing, so they replaced a wiring harness. Took it home and a few days later same thing. Now they want to replace the ECM. Is there something I can check before I drop more money for guess work?