Engine light, code P0131

I’ve pulled the code after my engine light came on, P0131, 02 sensor circuit low voltage bank one sensor. Took it to Christian Brothers and they got the same diagnosis and said it was my secondary sensor and that my catalytic converter was clogged and I needed a new catalytic converter with a quoted price of over $1,800.00. Am I delusional or are they trying to mess with me? That code is for the primary sensor before it even gets to the catalytic converter right? I have no idea how they came to that conclusion but wanted to get some feedback before I go talk to them.

Thank you,

Susan