Check engine light in 1999 Toyota Camry

We have a 1999 Camry with 130,000 miles that is in very good condition & runs well. A few months ago the check engine light came on. Our mechanic (who we think? is very good) said that the light was on to replace the oxygen sensor. Knowing that we don’t have loads of money & we have a newer Camry that we drive most of the time, he suggested that the sensor doesn’t have to be replaced. Our gas mileage would suffer, but it wouldn’t harm the engine.
We are happy to be able to save some money & we only drive the car on fairly short trips but we love the car & want to make sure that we are in fact not doing any damage by not replacing the oxygen sensor.
Any thoughts??
Thanks for your advice.

Most often it is the heater circuit that fails in the sensors. Your mechanic was correct that you can drive the vehicle like this without fear of damage just lower gas mileage. Depending on where you live this may have to be fixed to pass inspection for new tags.