Hello and welcome to Justanswer!

This code is for the left bank catalytic converter's efficiency. The converter monitors work by monitoring the switch rate between the upstream and downstream oxygen sensors. (upstream is before the cat, downstream is after) The engine controller knows that if the converter is working the downstream should switch from rich to center to lean much slower than the upstream. When the downstream sensor is switching at close to the rate of the upstream sensor it sets a failure code.

You've got one of two things going on. You either have a cat that has actually failed, or the upstream sensor on the left bank is getting lazy. As the sensors age they will start to naturally switch slower. At a certain point they will eventually set a code, but it's not at all unheard of for it to set a converter failure code before setting a code for being lazy.

If you have access to a full capability scan tool monitor the switch rates of the sensors. If you see the downstream is switching quickly you know the cat isn't working and will ned to be replaced. If the upstream isn't switching quickly, replace it with a Mopar sensor. If you don't have access to a scan tool I would start by replacing the left upstream sensor. It's far cheaper than a converter and a good start. It does happen once in a while on these, but converter failures on the Liberty aren't that common.