[Faisal_khan]

pawn Код:

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if (PRESSED(KEY_FIRE))
    {
        if(GetPlayerState(playerid) == PLAYER_STATE_DRIVER)
        {
            new str[60], vehicle = GetPlayerVehicleID(playerid);
            GetVehicleParamsEx(vehicle,engine,lights,alarm,doors,bonnet,boot,objective);
            if(engine != 1 && lights !=1)
            {
                SetVehicleParamsEx(vehicle,1,1,alarm,doors,bonnet,boot,objective);
                format(str, sizeof(str), "* %s accende il motore del veicolo", GetICName(playerid));
                SendNearByMessage(playerid, ACTION_COLOR, str, 10);
            }
        }
    }
    if (PRESSED (KEY_JUMP))
    {
        if(GetPlayerState(playerid) == PLAYER_STATE_DRIVER)
        {
            new str[60], vehicle = GetPlayerVehicleID(playerid);
            GetVehicleParamsEx(vehicle,engine,lights,alarm,doors,bonnet,boot,objective);
            if(engine == 1 && lights ==1)
            {
                SetVehicleParamsEx(vehicle,0,0,alarm,doors,bonnet,boot,objective);
                format(str, sizeof(str), "* %s spegne il motore del veicolo", GetICName(playerid)); /// this is the message that appears the first time I type /engine, even if the vehicle is turned off
                SendNearByMessage(playerid, ACTION_COLOR, str, 10);
            }
        }
    }
    return 1;
}

You were checking the KEY_JUMP inside the KEY_FIRE. And I have added a check for lights also.