With the general meaning of the U0121 Code, you cannot do anything. You need to retrieve the genuine meaning of the code what is related with the manufacturer information. The engine code suitable for the automobile engineer for solving the car engine. This code has dictionary meaning but that does not work. You can get that dictionary meaning from online. You can use Google to get the general meaning. However, read the manual of the manufacturer for your car and car engine. You must get sufficient information for solving the car engine problem. You have nothing to worry if you have given the car to professional automobile engineer.