The fault light of the Big Dipper engine is always on.

According to the analysis of the problems you mentioned, the working environment of automobile high-voltage lines is relatively bad. Under the condition of high voltage of about 20000V for a long time, the insulation layer of high voltage line is easy to break down or age, and it is difficult to confirm whether it is damaged only by appearance and measurement with multimeter. In the EFI system of the engine, the engine fault light will light up because of the leakage of the high-voltage line. Because the self-diagnosis system of the control unit generally does not prompt the fault code of the high-voltage line, it is difficult to find similar problems when they occur. I suggest you check this problem.

As for the problem of stuffy vehicles, it should be caused by too much carbon deposition in the combustion chamber. I suggest you consider spraying cleaning solution from the spark plug hole to clean the carbon deposit in the combustion chamber. This method is to use a good carbon remover, pump it into the combustion chamber from the spark plug hole in the form of foam with special equipment, melt the carbon deposits in the combustion chamber, and then suck out these liquid carbon deposits from the spark plug hole. You can know the cleaning effect from the color of the sucked substances, and check and adjust when disassembling the spark plug. This method is very good and does not disintegrate. At the same time, we should also check the dirtiness of carbon deposits in throttle, air intake, idle motor and other parts, and clean them up in time if they are too much.