Why can't Valin Hanma gearbox downshift?

The reasons why the downshift of Valin Hanma gearbox cannot be reduced are as follows: ① the throttle cable or throttle position sensor is improperly adjusted; ② The forced downshift switch is damaged or improperly installed; ③ The forced downshift solenoid valve is damaged or the line is short-circuited or open-circuited;

④ The forced downshift control valve in the valve plate is stuck.