All electronically controlled engines will save DTCs in ECU memory after sensor out of normal operating range. When problems monitored, they will be sent back to ECU, some serious problems will cause yellow or red diagnostic light on instrument panel lighten, when yellow light on, power and RPM of engine will reduce; when red light on, indicates the problem that sensor detected is serious. If ECU has stop control function, then this function will be triggered. Some electronic control system has diagnosis trigger switch, if operate this switch, check engine light on instrument panel will blink, for driver and maintenance technician to read flashing DTC. Some electronic control systems need use jumper to short circuit two terminals in diagnostic connector, to display DTC saved in ECU through flashing of diagnostic light of instrument panel. shows a 12-pin diagnostic data connector（DDL）, DDL connector normally located in truck cab(the specific position determined by manufacturer, this is a special case of Detroit DDEC I system or DDEC II system, but DDEC Ⅲ system and DDECⅣ system no longer adopt this kind of connector).
　　In order to flashing display DTC, ignition switch are required to turn off (in OFF) and insert jumper between terminal A and M which are marked clearly on connector. shows an example of interpreting flashing DTC, taking DDEC I system or DDEC II system as example, flash code 13 means coolant level sensor has detected coolant level too low, flash code 21 indicates throttle position sensor has detected circuit voltage too high.

　　Some vehicles adopt electronic instrument panel, which can directly read DTC of engine. Such as Dongfeng Tianjin, which can constantly displays running condition of engine and vehicle to driver, such as display Instantaneous fuel consumption in unit of KM/L. Although DTC is helpful to ascertain fault, if need further analysis of system DTC or fault part, diagnostic data readers used by different engine manufacturers are different, but some major diagnostic equipment manufacturers already can supply universal diagnostic tool that applies to different engines.
Such as Fcar auto diagnostic scanner.

——(pm fcar for link to website ~Jazz)