The engine problem occurs due to different reason and the B1991 Code may come when the car engine shows it. You need to fix the engine problem and then the code will be removed. That is why, you should fix the car engine. As the code has started with the B, the body of the engine has the problem and it is found from the dictionary meaning of the code. Do not depend on the dictionary meaning of the code. You need to get the real meaning and have all machines if you want to fix the car problem by yourself.