The P0140 only points to the rear O2 sensor. You could first try cleaning the connector in the engine bay and see if that helps, but if you have a lot of miles on the car I would suspect that it needs replacement.

Get an OEM replacement. Easy to identify, as it will have blue tape at all of the anchor points.

Here, in the rust prone part of the country, the most difficult part of the operation is removing the cross member under the car (4 bolts) which hides the O2 sensor. You can remove 3 of the bolts and just loosen one of the bolts on the passenger side and then slide the metal plate out of the way - makes it easier to reinstall.

If you do not clear the code, It could take a couple of engine starts to clear itself.
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Keeping it running is better than buying new