You need to solve the problem of the car engine and then the B1891 Code will disappear. This code comes when the car engine is diagnosed and the code gives the reason of showing the code. When the car engine is not diagnosed properly the area B1891, then the code comes. You can fix the issue from the engine with the automobile engineer. The problem should not be solved by yourself if you do not have any machine and experience of solving the car. You can hand over the car to the professional for solving the problem from the car engine.