an embodiment of the present invention will now be described with reference to accompanying drawings . fig1 is a diagram illustrating connection between the trouble diagnosing device according to this embodiment and an ecu mounted to the vehicle and designed to control the air suspension . this figure also shows a personal computer ( to be abbreviated as pc hereinafter ) 11 that constitutes the trouble diagnosing device . the pc 11 is connected to the ecu 13 mounted on the vehicle and serving as an electronic control device , via a vci ( vehicle communication interface ) 12 serving as an interface equipment and constituting the trouble diagnosing device . the ecu 13 includes an ecu for engine control , an ecu for air suspension , etc . the pc 11 and vc 112 are connected to each other via a multiple communication line 14 , and the vci 12 and ecu 13 are connected to each other via a multiple communication line 15 . an end of the multiple communication line 15 is coupled to a diagnosis connector 13 d of the ecu 13 . the vci 12 has a function of converting a communication specification used for the ecu 13 into a communication specification used for the pc 11 . the vci 12 has a built - in microprocessor and has a memory 12 m inside . as described above , the trouble diagnosing device itself is pre - equipped with the interface equipment serving as an interface with the electronic control unit mounted on the vehicle . thus , it is not necessary to particularly prepare a separate interface equipment . therefore , the operation for matching the specifications with each other can be omitted and therefore the increase in production cost can be suppressed . the ecu 13 has a diagnostic function . the diagnostic function means a self - diagnostic function . that is , the ecu 13 monitors input signals from various types of sensors , an actuator and switch , and when some abnormality occurs in anyone of the input signals , the ecu 3 stores the contents of the abnormality ( diagnosis code ) or the data at the time the abnormality occurred in the memory 13 m as maintenance and repair data . a predetermined area of the memory 13 m further includes a current trouble information region 13 a for storing current diagnosis codes , as shown in fig3 a and a past trouble information region 13 b for storing past diagnosis codes , as shown in fig3 b . in each of the current trouble information region 13 a and the past trouble information region 13 b , for example , eight diagnosis codes can be stored at the maximum . when a starter switch ( not shown ) is turned on to start the engine , diagnosis codes generated are stored one after another into the current trouble information region 13 a as current diagnosis codes in real time . when the starter switch ( not shown ) is turned off , the diagnosis codes stored in the current trouble information region 13 a are transferred to the past trouble information region 13 b of the memory 13 m , where the transferred codes are stored as past diagnosis code . after that , the current trouble information region 13 a is cleared to zero . as described above , the current trouble information region 13 a is capable of storing eight current diagnosis codes at the maximum . for example , in the case where three current diagnosis codes are already stored in the current trouble information region 13 a , and additional diagnosis codes that are the same as any of these three already stored codes are generated , the three already stored diagnosis codes are maintained as they are . in the case where eight current diagnosis codes are already stored in the current trouble information region 13 a , and a new current diagnosis code is generated , the oldest one of the already stored eight current diagnosis codes is deleted , and then the new current diagnosis code is stored in the current trouble information region 13 a . next , the structure of the pc 11 will now be described with reference to fig2 . fig2 shows a cpu ( central processing unit ) 21 . to the cpu 21 , a rom ( read - only memory ) 22 , a ram ( random access memory ) 23 , a key entry portion 24 , a touch - panel type display 25 having a rectangular display screen , an hdd ( hard disk device ) 26 , a communication i / f 27 and a printer i / f 28 are connected via a system bus 21 a . the hdd 26 stores a trouble diagnostic program for executing the diagnostic functions . ( 1 ) a main program as illustrated in fig4 ; and ( 2 ) a program for displaying an image corresponding to a touch button displayed on the display 25 when an operator touches the button , and a program for executing a process designated by the button . the details of these programs are as follows . for example , one of these corresponds to simultaneous display unit for displaying current diagnosis codes , past diagnosis codes , and a total number of each type of codes simultaneously on the display screen 31 . these data are displayed as a command for reading the current diagnosis codes and past diagnosis codes stored in the current trouble information region 13 a and past trouble information region 13 b , respectively , a command for reading the total number of the current diagnosis codes and past diagnosis codes , etc ., are output to the ecu 13 with the touch of a “ self - diagnosis ” button of the function menu , which will be described later . another program corresponds to a trouble information erasing unit for instructing the ecu 13 to erase certain diagnosis codes of at least one type of the current diagnosis codes and past diagnosis codes stored in the current trouble information region 13 a and past trouble information region 13 b , respectively , with the touch of an “ eraser ” button 43 . still another program corresponds to a printing unit for printing a presently displayed image by transmitting the data corresponding to an image presently shown on the display screen 31 to a printer ( not shown ) via the printer i / f 28 . still another program corresponds to a dialog - type trouble diagnosing unit for performing a dialog - type troubleshooting with the touch of a “ code - by - code troubleshooting ” button 42 . it should be noted here that the ram 23 has various work areas . next , the operation will now be described . the members are connected to each other as shown in fig1 . then , the pc 11 is turned on to start up the trouble diagnosing program , and then the starter switch ( not shown ) of the truck is turned on , the trouble diagnosis illustrated by the flowchart shown in fig4 is started . first , a startup image as shown in fig5 is displayed as the main menu on the display 25 ( step s 1 ). next , when the “ trouble diagnosis ” button is selected , then a system selection image is displayed on the display 25 , in which one of the three systems , namely , engine , chassis and body can be selected ( step s 2 ). next , when the chassis is selected from this image and then the air suspension is selected , the function selection image is displayed ( step s 3 ). this function selection image presents buttons for the “ self - diagnosis ”, . . . , “ calibration ”, . . . . when the “ self - diagnosis ” is selected from this image , what is shown in fig6 as a display screen 31 is displayed on the display 25 . in other words , when the “ self diagnosis ” button on the function display image is manipulated , the simultaneous display unit of the pc 11 transmits the command for reading the current diagnosis codes and past diagnosis codes stored in the current trouble information region 13 a and past trouble information region 13 b , respectively , via the vci 12 to the ecu 13 . the ecu 13 , when receiving this command , reads out the current diagnosis codes and past diagnosis codes stored in the current trouble information region 13 a and past trouble information region 13 b , respectively , and transmit them to the pc 11 . the pc 11 , when receiving these codes , counts the total number of the current diagnosis codes and past diagnosis codes transmitted . the display screen 31 illustrated in fig6 shows an example where three current diagnosis codes and one past diagnosis code are displayed . on the left - hand side of the display screen 31 , a current trouble information display section 32 which is capable of five displaying current diagnosis codes at the maximum is provided , whereas a past trouble information display section 33 which is capable of five displaying past diagnosis codes at the maximum is provided . more specifically , the current trouble information display section 32 is provided with five independent cells 32 a to 32 e in which trouble information are indicated , and similarly , the past trouble information display section 33 is provided with five independent cells 33 a to 33 e . a width a of each of the cells 32 a to 32 e and cells 33 a to 33 e is about 0 . 39 inch (= 1 cm ) to 0 . 79 inch (= 2 cm ) when the display screen has a size of 12 . 1 inches . as described above , when the display screen has a size of 12 . 1 inches ( the length of a diagonal line of the screen image is about 31 cm ), each of the cells 32 a to 32 e is displayed to have such a large width of about 0 . 39 inch (= 1 cm ) to 0 . 79 inch (= 2 cm ). with this size , if each cell of the trouble diagnosing device is manipulated by an operator in a maintenance and repair shop with work gloves on , the possibility of the operating error can be significantly reduced . in the case where the display screen 31 has a size of 15 inches , it is preferable that the width of each of the cells 32 a to 32 e should be set to about 0 . 49 inch (= 1 . 24 cm ) to 0 . 98 inch (= 2 . 48 cm ). in the case where the display screen 31 has a size of 17 inches , it is preferable that the width of each of the cells 32 a to 32 e should be set to about 0 . 56 inch (= 1 . 41 cm ) to 1 . 11 inches (= 2 . 82 cm ). thus , the width of each cell should preferably be set to about { fraction ( 1 / 30 )} to { fraction ( 1 / 15 )} of the length of a diagonal line of the screen image . dct indicated at the top section of the current trouble information display section 32 is the abbreviation of diagnosis trouble code , which means current diagnosis node . the contents shown in fig6 as the display screen 31 show an example in which the current trouble information display section 32 shows “ 11 ” as a current diagnosis code , “ common rail pressure sensor ” as the broken down part indicated by the code , “ 18 ” as another current diagnosis code , “ inomat system ” as the broken down part indicated by the code , “ 25 ” as still another current diagnosis code , “ vehicle speed sensor ” as the broken down part indicated by the code . further , the past trouble information display section 33 shows “ 32 ” as a past diagnosis code , and “ boost sensor ” as the broken down part indicated by the code . as described above , when the “ self diagnosis ” button of the function display image is manipulated , current diagnosis codes and past diagnosis codes are indicated in the current trouble information display section 32 and the past trouble information display section 33 , respectively , at the same time on the display screen 31 so that they can be compared and contrasted with each other . in this manner , troubles that occurred in the past and those occurred this time can be easily compared with each other . thus , it is possible to recognize the past troubles and current troubles at a glance without switching the images during the repair . therefore , the ease of repair can be improved . further , the current trouble information display section 32 includes in its lower portion a total trouble count indicating section 34 a that indicates the total number of current diagnosis codes . similarly , the past trouble information display section 33 includes in its lower portion a total trouble count indicating section 34 b that indicates the total number of past diagnosis codes . the total number of current diagnosis codes indicated in the total trouble count indicating section 34 a and the total number of past diagnosis codes indicated in the total trouble count indicating section 34 b can be obtained by counting , on the pc 11 side , the number of current diagnosis codes and the number of past diagnosis codes , respectively , which have been transmitted from the ecu 13 . according to the embodiment , the total number of each group of the present and past diagnosis codes is indicated , and therefore it is possible to know whether there are more data present that cannot be displayed in the current trouble information display section 32 and past trouble information display section 33 . thus , it is very hard to miss even those diagnosis codes that are not displayed in the image screen . further , the total number of each of the current and past diagnosis codes is displayed , and therefore it is possible to know how serious is a trouble at a glance , and to know the frequency of troubles of the ecu 13 . for example , in the example shown in the figure , three current diagnosis codes are indicated in the current trouble information display section 32 , and the count indicated in the total trouble count indicating section 34 a is “ 3 ”. therefore , in this case , it is possible to understand that all of the current diagnosis codes are presently indicated in the current trouble information display section 32 . suppose here the case where five current diagnosis codes are indicated in the current trouble information display section 32 and the count indicated in the total trouble count indicating section 34 a is “ 8 ”. in this case , three current diagnosis codes remain not displayed . in order to display these three remaining current diagnosis codes on the current trouble information display section 32 , the data can be scrolled on the screen by appropriately manipulating the cursor buttons 35 and 36 provided on the right - hand side of the current trouble information display section 32 . cursor buttons 37 and 38 designed to scroll displayed data are displayed on the right - hand side of the past trouble information display section 33 . thus , with the total number of each group of the current and past diagnosis codes displayed , it is possible to know how serious is a trouble at a glance , and to know the frequency of troubles of the ecu 13 as a whole . further , the section of the menu bar provided in the lowermost section of the display screen 31 displays the display button 41 for printing the contents presently shown on the display screen 31 , the “ code - by - code troubleshooting ” button 42 for performing a dialog - type troubleshooting , and the “ eraser ” button 43 for erasing certain diagnosis codes of at least one group of the current diagnosis codes and past diagnosis codes stored in the current trouble information display section 32 and past trouble information display section 33 , respectively . for example , when the printing button 41 is manipulated by touching it , the contents of the current display screen 31 are printed and output from a printer ( not shown ) connected the printer i / f 28 . thus , the current diagnosis codes and past diagnosis codes are printed at the same time on the same sheet , the record of maintenance and repair can be easily and accurately formed . further , for printing , it suffices only if the printing button 41 is touched , and thus the printing order can be easily made . furthermore , by having the current trouble data and past trouble data on a print , the data can be accurately passed on to another mechanic who next takes case of the vehicle , thereby making it possible to improve the efficiency of the maintenance and repair . moreover , when one of the cells 32 a to 32 e in which current diagnosis codes or past diagnosis codes are displayed is selected by touching it , and then the “ code - by - code troubleshooting ” button 42 is touched , the dialog - type trouble diagnosis corresponding to the selected diagnosis code is started . in other words , under what conditions the selected diagnosis code occurred is indicated on the displayed image 31 , and the maintenance and repair data stored in the memory 13 m of the ecu 13 are displayed . thus , with these displayed data , the inspection procedure can be carried out in a dialog manner . as described above , an operator can easily take care of the trouble without referring to the maintenance and repair guidebook . on the other hand , when at least one of the current trouble information display section 32 and past trouble information display section 33 is selected by touching it , and then the “ eraser ” button 43 is touched , the data of the selected one of the current diagnosis codes and past diagnosis codes are erased from the memory 13 m . in this manner , for example , when the cause for a trouble is investigated and the troubled part is repaired in a maintenance and repair shop , the current diagnosis codes and past diagnosis codes can then be erased to start a new self - diagnosis . in case where a vehicle with a trouble is brought in a maintenance and repair shop , but the trouble is not detected as current trouble information , such a conduct that easily causes a trouble as shaking a part to be inspected is performed . if trouble information corresponding to the inspected part is detected as current trouble information after this conduct , it can be judged that the inspected part is broken . thus , it is possible to carry out the investigation of the cause for a trouble . it should be noted here that it is alternatively possible to print out those current diagnosis codes and past diagnosis codes that are not displayed in the display screen 31 when the printing button 41 is touched , as well if there are any . in the above - described embodiment , the total number of each group of the current diagnosis codes and past diagnosis codes transmitted from the ecu 13 is counted on the pc 11 side ; however it is alternatively possible to count the total number on the ecu 13 side , and transmit it to the pc 11 . the renewal manner of the current diagnosis codes stored in the current trouble information region 13 a of the memory 13 m and the past diagnosis codes stored in the past trouble information region 13 b , which was described with reference to fig3 a and 3b is not limited to that of this embodiment , but it can be modified into various versions . it should be noted that in the above - described embodiment , the programs that correspond to various units are stored in the hdd 26 ; however the present invention is not limited to this , but it is alternatively possible to store these programs in an external storage means such as a cd - rom or fd , and download them to the hdd 26 of the pc 11 as needed . further , in the above - described embodiment , a touch panel - type display is used ; however in the case of a pc that does not equipped with a touch panel - type display , each operation can be assigned with use of a mouse . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .