You should test the car engine to know the reason of the engine P0378 Code. There are some symptoms what you need to check to identify the real cause of the car engine problem. These symptoms Decreased engine performance and engine may not start are relevant with the car engine problem. If the real meaning of the code is not known, you cannot fix the car engine problem. Due to use of the wrong meaning of the code, new problems may started in the car engine. If you do not test the car engine, you cannot identify the problem and you cannot take step for solving the car engine problem.