6.4.3. System of onboard self-diagnostics (OBD) - the general information

General description

Problem of any onboard system of self-diagnostics (OBD) is identification of refusals and violations of functioning of under control systems with entering in the processor memory of the corresponding diagnostic codes (DTC) and the notification of the driver about the violation fact (usually by means of the control lamp of refusals of MIL/" which is built in in a combination of devices check the engine").

Besides the DTC code in memory of ECM it is fixed also current working parameters of the engine at the time of violation identification.

At violation of serviceability of functioning of the information sensors which are taking part in process of management of the engine, ECM can make switching of systems in emergency operation. At the same time the basic working parameters ensuring adequate functioning of the engine (some absolute pseudo-signal of the faulty sensor is feigned directly by the module of management), however with inevitable decrease in efficiency of its return and increase in fuel consumption are activated, - the car should be driven away on car repair shop for the purpose of identification and elimination of causes of failure.

Several diagnostic devices making monitoring of separate parameters of systems of decrease in toxicity and fixing the revealed refusals in memory of the onboard processor in the form of individual codes of malfunctions are a part of the OBD system. The system makes also check of sensors and actuation mechanisms, controls operational cycles of the vehicle, provides a possibility of freezing of parameters and cleaning of the block of memory.

The models considered in the present manual are completed with system of onboard diagnostics of the second generation of the SAE standard (OBD II). A basic element of any OBD system is the onboard processor more often called by the electronic module of management (ECM). ECM is a brain of a control system of the engine. Basic data, and also signals of the registered OBD of refusals and violations arrive on the module from various information sensors and other electronic components (switches, the relay, etc.). Codes of the registered malfunctions are fixed in the processor memory.

The function of self-diagnostics with feedback allowing to minimize effect of errors of the main equipment of entrance and output paths of ECM is provided in modules of management of diesel models.

The principle of the organization of function of self-diagnostics of ECM on diesel models

Data read-out of the processor memory of OBD is made by means of the special scanner (HI-Scan type) connected to the 20-contact diagnostic socket of reading of the database (DLC) fixed on the air cleaner case in the left back corner of a motive compartment - one more standard 16-contact DLC OBD II is established in interior of the car under the dashboard at the left.

Diagnostic DLC sockets allow to make reading of codes of malfunctions by means of the special scanner

1 — Grounding
2 — Alarm grounding
3 — Function of onboard diagnostics
4 — Tension of the battery (B)

Special guarantee certificates with the extended validity extend to service of components of control systems of engine/decrease in toxicity of the fulfilled gases. It is not necessary to make attempts of independent performance of failure diagnostics of ECM or replacement of components of system, to an exit of terms of these obligations, - address specialists of company car repair shops of the KIA company.

Data on diagnostic units

Check of serviceability of functioning of components of control systems of the engine and decrease in toxicity of the fulfilled gases can be made by means of the universal digital measuring instrument (multimeter). Use of the digital measuring instrument is preferable for several reasons. First, on analog devices it is rather difficult (sometimes, it is impossible), to define result of the indication to within the 100-th and thousand shares while at inspection of the contours including electronic components in the structure, such accuracy is of particular importance. The second, not less important, the fact that the internal contour of a digital multimeter, has rather high impedance is the reason (internal resistance of the device makes 10 megohms). As the voltmeter is connected to the checked chain in parallel, the accuracy of measurement of subjects is higher, than smaller parasitic current will pass through actually device. This factor is not essential at measurement of rather high values of tension (9 ÷ 12 V), however becomes defining at diagnostics of the elements giving low-voltage signals, such as, for example, lambda probe where it is about measurement of shares of volt.

Use at diagnostics of the considered systems of a digital multimeter with a high impedance significantly increases the accuracy of the measurements performed in the low-voltage range.

Parallel monitoring of parameters of signals, resistance and tension in all chains of management can be made by means of the splitter connected consistently to the socket of the module of management (ECM). Measurement of parameters of signals on splitter plugs in various modes of functioning of the engine allows to define current state of the last and to reveal the violations taking place.

At diagnostics of electronic control systems of the engine, transmission, ABS and SRS special scanners of the SAE (GST) standard to which number also the HI-Scan scanner belongs are used. Many SAE scanners of the second generation (OBD II) are multipurpose at the expense of a possibility of installation of replaceable cartridges depending on model of the diagnosed car (Ford, GM, Chrysler, etc.), others are tied to requirements of the regional authorities and intended for use in certain regions of the world (Europe, Asia, the USA, etc.). Connection of the scanner is made to onboard DLC.

Alternative way of data read-out of OBD is connection to system of the personal computer equipped with a special cable and equipped with the software of OBD.

Data read-out by means of scanners can find more detailed information on the websites www.obdii.com, www.obd-2.com and www.obd-2.de. The free version of a browser OBD-II can be downloaded from the website of authors of the present manual www.arus.spb.ru.

The universal K-L-Line adapter serves for coordination of signals of RS-232 port and ISO-9141 interfaces (K-Line) and ALDL. Various cables allowing to make data read-out of OBD from cars of various brands can be connected to sockets of the adapter. The switches and elements of indication provided on the adapter allow to choose necessary operating modes and to approximately estimate quality of functioning of output lines. So, the luminescence of a green light-emitting diode with marking of L-Line demonstrates connection of the line L with the mass of the car. Activation of a red light-emitting diode with marking K-Line confirms about presence on the line K to time of high potential at the moment. At the established connection with the OBD system of the car the blinking of indicators can be imperceptible for an eye in view of the high speed of data exchange. Connection to the computer is made directly in 25-contact SOM port or in 9-contact SOM port by means of the transitional RS232 25-9 cable.

Universal K-L-Line adapter.

Some readers besides usual diagnostic operations allow at connection to the personal computer to make printing of the schematic diagrams of various equipment which are stored in memory of the module of management (if those are put in ECM), to program anticreeping system and control units of various devices of the car, and also in real time to observe signals in electric chains of the car.

Reading of the DTC codes

More detailed information on reading of codes of malfunctions is provided in the user's guide to the scanner. The list of codes of malfunctions is provided in Specifications to the Head of the Power supply system, management of engine/decrease in toxicity of the fulfilled gases and production of the fulfilled gases.


  1. Connect the reader HI-Scan to any of DLC.
  2. Start the engine.
  3. Working according to attached to the scanner by instructions, make reading of the OBD codes of 5-digit DTC brought in the processor memory. The list of separate codes see in Specifications of the Head of the Power supply system, management of engine/decrease in toxicity of the fulfilled gases and production of the fulfilled gases.
  4. Make the corresponding recovery repair, then clear the processor memory.

Cleaning of memory of system of self-diagnostics

Alternatively cleaning of the processor memory of OBD II can be made by means of the scanner connected to DLC.


  1. For a while more than 20 seconds disconnect a negative wire from the battery.
  2. After connection of the battery connect the reader to DLC and include ignition.
  3. Start the engine and let's it work about three minutes with turns of 2000 in a minute.
  4. Make sure of absence in the processor memory of codes of malfunctions (DTC).
  5. In case of need make the corresponding recovery repair and repeat the procedure of cleaning of memory.

Connection of the personal computer to onboard system of self-diagnostics of OBD II by means of the interface BR16F84-1.0 controller under protocols of the SAE standards (PWM and VPW) and ISO 9141-2

The controller is not intended connections to onboard systems of self-diagnostics of the first generation (OBD I)!

Models of production of the company GM, PWM - Ford, to ISO 9141-2 - Asian and European models meet the VPW standard.

General information

The scheme of the organization of connection of PC to the diagnostic DLC socket of onboard system of self-diagnostics of OBD II by means of the BR16F84-1.0 controller

The considered device represents the microcontroller executed on KMOP (CMOS) technology. The device plays a role of the elementary scanner and is intended for reading of diagnostic codes and data of the OBD II system (engine turns, temperature of cooling liquid and the soaked-up air, load characteristics, a consumption of the air coming to the engine, etc.) within the J1979 specification of the SAE standard via the tire of any execution (PWM, VPW and ISO 9141-2).

Main destination

For connection to the computer of rather 3-wire wire, connection to the diagnostic socket is carried out by a 6-wire wire. Supply voltage moves on the controller via the 16-contact diagnostic OBD socket.

Recommendations about application

For connection of the device to the car the unscreened cable, no more than 1.2 m long can be used that has special value when using the PWM protocol. When using a cable of bigger length it is necessary to reduce resistance of resistors on an entrance of the device (R8 and R9 or R15). When using a screened cable, the screen should be switched-off for the purpose of decrease in capacity.

The cable for connection to a serial port of the computer can also be unscreened. The device steadily works with a cable up to 9 m long. With much bigger length of a cable it is necessary to use more powerful communicator of RS 232.

The topology of electric connections is any. At the increased humidity use the additional shunting condensers.

The free software for reading of codes and data can be downloaded from the websites of producers, or the website of arus.spb.ru publishing house and is intended for use under DOS. The insignificant size of a software application in option "under DOS" allows to accomodate it on the loading diskette DOS and to use even on the computers equipped with the software, incompatible with DOS. An optional condition is even existence in the computer of the hard drive.

General principles of data exchange

If opposite is not stipulated especially, all numbers are given in a 16-richny format (hex), - the decimal format is designated by dec tag.

Data exchange goes on three-wire consecutive connection without application of initsializatsionny exchange of office messages (handshaking). The device wiretaps the channel on existence of messages, executes the accepted commands and transfers results to the personal computer (PC) then immediately comes back to the listening mode. The data entering the controller and proceeding from it are organized in the form of a chain of the bytes which are consistently going one after another, first of which is control. Usually control byte represents the number from 0 to 15 dec (in decimal calculation) (or 0-F hex) describing the number of the information bytes following further. So, for example, the 3-byte team will look as follows: 03 (control byte), 1st byte, 2nd byte, 3rd byte. The similar format is used as for the entering teams on poll of onboard system of self-diagnostics, and for the outgoing messages containing the requested information. It is necessary to notice that in control byte only four younger bits are used, - the senior bits are reserved under some special teams and PC at initialization of connection with the controller and coordination of the protocol of data transmission can be used, and also the controller for control of errors of transfer. In particular, in case of a mistake by transfer, the controller makes installation of the senior meaning bit (MSB) of control byte in unit. By successful transfer all four senior bits are established in zero.

There are separate exceptions to the rules of use of control byte.

Initialization of the controller and onboard system of self-diagnostics

To start data exchange of PC has to make installation of connection with the controller, then initialize the controller and the channel of data of OBD II.

Connection installation

After connection of the controller to PC and the diagnostic OBD socket its initialization for the purpose of prevention of the "lags" connected with noise in consecutive lines has to be made in case their connection was made before inclusion of power supply of the controller. The simplest check of activity of the interface is at the same time made. First of all the one-byte signal of 20 hex perceived by the controller as the command for connection installation is sent. In reply the controller instead of control sends the only byte of FF hex (255 dec) and passes into a waiting mode of reception of data. Now PC can pass to initialization of the channel of data.

This case is one of the few when the controller does not use control byte.


At this stage initialization of the protocol under which data exchange will be made, and in case of the ISO protocol – initialization of onboard system is made. Data exchange is made on one of three protocols: VPW (General Motors), PWM (Ford) and ISO 9141-02 (Asian/European producers).

There is a set of exceptions: so, for example, at poll of some models of the Mazda cars the "Ford" PWM protocol can be used. Thus, at emergence of problems of transfer it is necessary first of all to try to use any other protocol. The choice of the protocol is made by transfer of the combination consisting of control byte of 41 hex and the byte defining protocol type following directly it: 0 = VPW, 1 = PWM, 2 = ISO 9141. So, for example, at the command of 41 02 hex initialization of the ISO 9141 protocol is made.

In reply the controller sends control byte and byte of a state. The MSB installation of control byte speaks about existence of problems, at the same time the byte of a state following it will contain the relevant information. At successful initialization the control byte of 01 hex indicating that the verification byte of a state follows further is sent. In case of the VPW and PWM protocols the verification byte represents a simple echo of the byte defining the protocol (0 or 1, respectively), at initialization of the ISO 9141 protocol it will be the digital key returned by the onboard OBD processor and defining what of two versions of the protocol which are slightly differing from each other will be used.

The digital key has purely information appointment. It is necessary to notice that initialization of the VPW and PWM protocols happens much quicker as demands only transfer of the relevant information to the controller. On the models meeting ISO, initialization takes about 5 seconds spent for the information exchange of the controller with the onboard processor made with a speed of 5 baud. It is necessary to draw the attention of the reader that on some models of cars of the ISO family 9141 initialization of the protocol stop if the request for issue of data is not transferred during a 5-second interval, - told means that PC has to make automatic delivery of inquiries each several seconds, even in the single mode.

After installation of connection and initialization of the protocol regular data exchange, consisting of the inquiries arriving from PC and the answers issued by the controller begins.

Data exchange order

Functioning of the controller when using protocols of the ISO 9141-2 and SAE family (VPW and PWM) happens according to a little various scenarios.

Exchange under the SAE protocols (VPW and PWM)

At data exchange under these protocols there is a buffering of only one shot of data that means need of a specification subject to capture or return of a shot. The onboard processor can transfer the packages consisting more than of one shot in some (rare) cases. In such situation the inquiry has to repeat until all shots of a package are not accepted.

The inquiry is always formed as follows: [Control byte], [Inquiry according to the SAE standard], [Number of a shot]. As it was already mentioned above, the control byte usually represents the number equal to the full number of the following bytes behind it. The inquiry is made out according to the SAE J1950 and J1979 Specifications and consists of heading (3 bytes), the sequence of information bytes and byte of control of a mistake (CRC). Let's notice that while information on inquiry is formed in strict compliance with the SAE Specifications, a consumer of control byte and number of a shot is the interface controller.

At a successful completion of the procedure the response message always has the following format: [Control byte], [The answer according to the SAE standard]. The control byte, as well as earlier, defines the number of the information bytes following it. The answer according to requirements of the SAE standard consists of heading (3 bytes), a chain of information bytes and byte of CRC.

At failure the 2-byte response message is sent: [Control byte], [State byte]. At the same time in control byte the MSB installation is made. Four younger bits form number 001 demonstrating that control is followed by the only byte - state byte. This situation can arise rather often as Specifications allow a possibility of a lack of distribution the onboard processor of data, and also transfer of incorrect data in case the inquiry does not conform to the standard supported by producers of the car. Also the situation when required data are absent in random access memory of the processor of time at the moment is possible. When the controller does not receive the expected answer, or obtains the damaged data, the MSB installation of control byte is made, and after control the state byte is given.

At collisions in the tire the interface develops the only byte of 40 hex which is control byte with the nullified younger bit. The similar situation can arise rather often when loading a car tire messages of higher than at diagnostic data of a priority, - the computer has to repeat initial inquiry.

Exchange under the ISO 9141-2 protocols

ISO 9141-2 is used by most of Asian and European producers of automotive vehicles. The structure of the formed PC of inquiry a little in what differs from used in the SAE standards, with only that difference that the controller does not need information on number of a shot and the relevant data should not be present at a package. Thus, the inquiry always consists of control byte and the chain of the information bytes including checksum following it. As the response message the controller just relays the signals created by the onboard processor. The control byte in the response message is absent therefore PC perceives the arriving information a continuous stream until the chain is not interrupted by the pause in 55 milliseconds reporting about end of information package. Thus, the response message can consist of one or more shots according to requirements of the SAE J1979 specifications. The controller does not make the analysis of shots, does not reject not diagnostic shots, etc. PC has to make by own forces processing of the arriving data with the purpose of exarticulation of separate shots by the analysis of heading bytes.

Responses to the majority of inquiries consist of the only shot.

The modifications made in the interface controllers of the latests version

All information bytes are transferred in a 16-richny format (hex).

The symbol XX means the uncertain, reserved or unknown byte.

The main differences of process of data transmission under the SAE and ISO 9141 protocols, characteristic of the interface controllers of the latests version, and also a data transmission order under the ISO 14230 protocol are given below:

Connection installation

The order of installation of connection did not change:

Sending: 20
Reception: FF

Choice of the protocol

The protocol is chosen in as follows:

  • Sending:
41, 00
  • Reception:
02, 01, XX
  • Sending:
41, 01
  • Reception:
02, 01, XX
SO 9141:
  • Sending:
42, 02, adr, where: adr - address byte (usually 33 hex)
  • Reception:
02, K1, K2, where K1, K2 - key bytes of ISO
  • Or:
82, XX, XX (error of initialization of ISO 9141)
ISO 14230 (bystry initialization):
  • Sending:
46, 03, R1, R2, R3, R4, R5, where: R1 ÷ R5 - the message on the beginning of request of ISO 14230 on connection installation, usually R1 ÷ R5 = C1, 33, F1, 81, 66
  • Reception:
S1, S2, … … …, where S1, S2, … … … - the message on the beginning of the response of ISO 14230 to connection installation

More than one ECU can consistently be transferred. As the answer the negative code of the answer can be used.

Typical affirmative answer looks as follows: S1, S2, … …. = 83, F1, 10, S1, E9, 8F, BD
ISO 14230 (slow initialization): Similar to ISO 9141

Remark and comments

If use of the controller for data transmission only on any to one is planned or to two of protocols, excess components can be excluded. For example, at the organization of the scheme under the VPW (GM) protocol in a wire of connection of the controller to the car only three veins of an electrical wiring (plug 16, 5 and 2) will be required.

If the PWM protocol is not used, the R4, R6, R7, R8, R9, R10, T1, T2 and D1 elements can be excluded.

At refusal of exchange under the ISO protocol elements are subject to an exception: R15, R16, R17, R18, R19, R21, T4 and T5.

The refusal of use of the VPW protocol allows to exclude the following elements: R13, R14, R23, R24, D2, D3 and T3.

Coal and film resistors with the 5 percent admission of resistance are used.

Pay attention to lack of the button of emergency reset (RESET), - in case of need such reset can be made by a controller detachment from the automobile socket (reset of the interface processor will happen automatically). Restart of the software on PC leads to repeated initialization of the interface.