Since the 1980s, major automobile manufacturers such as the United States, Japan, and Europe have begun to equip their EFI vehicles with OBD, but the initial OBD has no automatic inspection function, and can only monitor the basic conditions of the sensor’s open/short circuit. . It cannot monitor the accuracy of the sensor. The fault light will only illuminate when the sensor has completely failed. Since each of the major automobile manufacturers uses their own design diagnostics (requires a dedicated code reader) and a custom fault code, this brings great inconvenience to the maintenance inspection. The OBD-II, which is more advanced than OBD, was produced in the mid-1990s. The Society of Automotive Engineers (SAE) has developed a set of standard specifications that require automotive manufacturers to provide a unified diagnostic model in accordance with OBD-II standards. The II interface provides information on the power system, site system, body system, and signal system.

OBD-II features:

1. The shape of the unified vehicle diagnostic seat is 16PIN.


3. Unify the same fault codes and meanings of each vehicle type.

4. With driving recorder function.

5. It has the function of redisplaying the memory fault code.

6. Has the function of clearing the fault code directly by the instrument.

Although OBD-II is very effective in monitoring vehicle emissions, it is “conscious” for drivers to accept warnings. For this reason, OBD-III, which is more advanced than OBD-II, has been produced. The main purpose of OBD-III is to integrate the detection, maintenance and management of automobiles to meet the requirements of environmental protection. The OBD-III system will enter the engine, gearbox, ABS and other system ECUs (computers) to read the fault code and other related data, and use the small vehicle communication system, such as GPS navigation system or wireless communication method to identify the vehicle’s identity code. The information such as the fault code and the location is automatically notified to the management department. The management department issues instructions according to the level of the vehicle’s emission problem, including suggestions for where to repair, time limits for solving the problem, etc., and for those who violate the time limit. The vehicle issued a no-go command. Therefore, the OBD-III system not only warns the driver of vehicle emissions, but also penalizes the offender.


Summary: OBD generation can only sense that the sensor is completely broken, and the standard protocol is not uniform.

The OBD II agreement is a unified standard, but it is not absolute. For example, the site system and the body system are not uniform.

Three generations of OBD: reverse control is possible, and the protocol is highly unified

OBD second generation working principle:

If you open it, you will find that the diagnostic line is connected to the ECM (or ECU, engine control unit) (CAN line is the gateway). OBD is not a very independent system on physical hardware, it just provides an interface to the engine management system (and the entire vehicle control system)

In some mid- to high-class cars, not only the ECU is applied to the engine, but also the ECU can be found in many other places. For example, anti-lock brake systems, four-wheel drive systems, electronically controlled automatic transmissions, active suspension systems, airbag systems, multi-directional adjustable electronically controlled seats, etc. are all equipped with their own ECUs. As the electronic automation of cars increases, ECUs will increase and the lines will become increasingly complex. In order to simplify the circuit and reduce the cost, the information transmission between multiple ECUs in the car adopts a technology called multiplex communication network, which forms the ECU of the whole vehicle into a network system, that is, the CAN data bus.


OBD can read two types of data:

1. CAN bus data: vehicle speed, various temperatures, wheel speed, engine torque, throttle, brake pedal, gear lever position, air conditioning and other equipment work, fault information … very much. The characteristics of these signals are generally: the data used by different control modules.

2. Controller internal data: CAN bus provides you with a channel for data circulation. Through this channel, the acquisition device sends instructions to the relevant controller, and the controller returns relevant data. In this way, various variables in the controller software can be collected, so that almost all data can be collected. Because there are more actions to send commands, it is more resource intensive. The richness of this kind of data is far far greater than the former. But only the developer can know how to configure to collect this data, non-developers still don’t think about it, unless someone leaks it to you.
Obd encoding rule

1, the value of the fault code data is the same as the chicken ribs

The main function of OBD-II is to provide fault code diagnosis. When there is a discharge fault, the ECU records the fault information and related codes and issues a warning through the fault light. In general, the OBD-II fault code consists of one letter and four digits. As shown in the picture below, most OBD manufacturers can only obtain the general fault code information of “PO/P2/P3400-P3FFF, other fault code information customized by the car manufacturer, only the original factory can diagnose. For example The Volkswagen model has 15,000 fault codes, and the general fault code is less than 3,000, accounting for only one-fifth, which is far from meeting the requirements of vehicle diagnosis.

2. Frequent diagnostic data is unstable

OBD collects data using “question-and-answer”, that is, the way OBD communicates with ECU is a question-and-answer method. The OBD asks the engine ECU that if the ECU has data, it will feed back to the OBD. If there is no data, there is no feedback or feedback fixed code. Vehicle fault scanning has its own rules. It is not possible to scan the fault code every second. Otherwise, the vehicle will have a dashboard fault light or the engine board will burn out due to too frequent scanning of the vehicle system.

3. Private protocol can crack and update

Post time: Sep-02-2019