Ask them to prove to you it's broken.
It could be the engine, the fuel injection, electrical, the cat itself or the sensor, ask them to explain which one it is and why.
Is the sensor actually bad or is it giving a reading it shouldn't?
Are they going on a simple error code, or have they tested the sensor?
Is your check engine light on?
In short do not beleive any diagnosis unless you are 100% sure that the part has actually failed, and somebody has shown you they are responding to more than just a computer error code, and have actually verified this or that has failed.