Not finding a U0121 code in the Cruze shop manual, U codes deal with power management and data links. In the Cruze, can be randomly set due to God knows what, and should go away after about four ignition systems.
Ready erroneous codes with a generic scanner has been a long term problem with GM, need a GM Tech II scanner to get the correct code, believe they do this to sell more scanners. One thing I never liked about software, you can lie and get away with it.
Use to be all electronic modules were isolated from each other and just the ECU, added a bunch connected with a data link, even the radio is part of it, so they are all talking to each other, some aren't listening.
Besides this code, are you experiencing other symptoms? Knowing the symptom is far more important than the codes.
I’m sorry to hear that you are experiencing concerns with your Cruze. Your local certified Chevrolet dealership would be more than happy to perform a diagnosis and recommend a resolution. If you need any assistance locating or getting in contact with your dealer, please feel free to send us a private message. We are always happy to help.