OBD2 [On-Board Diagnostics II] defines a communications protocol and a standard connector to acquire data from passenger cars. OBD was required by U.S. EPA on all gasoline powered cars and light duty trucks manufactured for the U.S. after 1996 to help monitor/inspect vehicle emissions [as required by the Clean Air Act Amendments of 1990].
OBDII is the second generation of the OBD specification. The first generation OBD monitored fewer emission related components, was not calibrated to a specific level of emission performance, and was not as fell defined. Any passenger cars and truck produced after 1996 uses the OBD II standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.
OBDII will light a warning lamp called a MIL (malfunction indicator lamp), also known as the "check engine" light on the dash. A scan-tool may also be used to probe the OBDII connector OBDII data as defined by the SAE J1979 standard. The warning light may come on for any number of reasons and manufacturers recommend having the vehicle serviced as soon as possible. However; the Check Engine light could also come on for such simple reasons as filling the tank while the vehicle is running, or leaving the gas cap off. It may take up to three days for the light to go back off, after coming on for a missing gas cap.

J1962 - SAE (Society of Automotive Engineers) standard defining the physical connector used for the OBDII interface.
J1850 - SAE standard for the Class B Communications Network Interface (standard defines the actual J1850 signaling and timings)
J1939, ISO 11898
J1978 - SAE standard for OBD II scan tools
J1979 - SAE standard for diagnostic test modes
J2012 - SAE standard for EPA emission test report format.
J2178-1 - SAE standard for Class B Communications Network Message: Detailed Header Formats and Physical Address Assignments
J2178-2 - SAE standard for Class B Communications Network Message: Data Parameter Definitions
J2178-3 - SAE standard for Class B Communications Network Message: Frame IDs for Single Byte Forms of Headers
J2178-4 - SAE standard for Class B Communications Network Message: Message Definitions for Three Byte Headers
Code of Federal Regulations [CFR]: 40 CFR; 86.094-17h

OBD stands for On-Board Diagnostics. OBD-II (OBD two) is the latest industry standard, which was introduced on 1996 vehicle models. It provides monitoring of nearly all engine controls, and also some other parts of the vehicle (chassis, body, etc.).

The most useful part of OBD to most drivers is the ability to read information from the vehicle. You can read trouble codes, and even monitor real-time statistics such as RPM, vehicle speed, etc. There are a variety of different devices on the market ranging from basic code scanners to advanced displays that calculate MPG as you drive. Check out the Links section of this site for more information on available products.

For information on what an OBD code is, go to this FAQ article..

http://www.obd-codes.com/

OBD-2 universal cable scheme forPWM,VPW, andISO 9141-2 vehicles pinout

This device is a CMOS microcontroller which is designed to interface a personal computer or laptop with a vehicle"s On Board Diagnostic (OBD II) interface. It is intended to function with all three of the protocols used by vehicle manufacturers to implement the OBD II system as defined by SAE and ISO specifications. The OBD II system became mandatory for 1996 and up vehicles, but some vehicles were already fully or partially compatible with OBD II requirements prior to 1996. The chip is not suitable for the earlier vehicles such as OBD I. The device is intended to function as a simple scan tool and is capable of sending and receiving any OBD II message defined in SAE J1979 for any of the three types of OBDII bus implementations (PWM, VPW, ISO 9141-2). It can also be used as an inexpensive interface for custom instrumentation monitoring various vehicle parameters such as speed, RPM, coolant or intake air temperature, engine load, intake air flow rate, etc.

Components:

C5, C6  .01 UF, 25V
C4      .1 UF, 25V 
C3      .47 UF, 50V 
C2,C1    20 pf, 50V 
R11, R12, R23, R9, R1, R3, R4, R25, R8  10 KOhm, 1/4 Watt 
R24 18 KOhm, 1/4 W 
R19, R18, R7, R20, R22, R5, R6, R13 1 KOhm, 1/4 W 
R21 5.6 KOhm, 1/4 W
R16, R17 510 Ohm, 1/4 W
R10 62 KOhm, 1/4 W
R15 9.1 KOhm, 1/4 W
R14 270 Ohm, 1/2 W
R2   - Not used  
D2  1N4739 Zener, 9.1V, 1W 
D1, D3  1N4148 Diode
Q3, Q4, Q5 PN2222 Transistor, NPN
Q2 PN3904 Transistor, NPN
Q1 PN3906 Transistor, PNP
IC2 7805 Regulator, 5V
IC3 LM339N Quad Comparator 
IC1 16F84-20 Microprocessor (firmware 1.07)
XTL 1 20 MHz Crystal, 20 Mhz

If you don"t use PWM, you can omit R4, R6, R7, R8, R9, R10, Q1, Q2, D1.
If you don"t use ISO, you can omit R15, R16, R17, R18, R19, R21, Q4, Q5
If you don"t use VPW, you can omit R13, R14, R23, R24, D2, D3, Q3

Don"t connect external power sources - all power direved from pin 16 of OBDII

Serial port works at 19200 baud

Scheme is unuseful without .hex for IC1 firmware 1.07...

Can be replaced using scheme with ELM327 chip (http://www.elmelectronics.com/DSheets/ELM327DS.pdf). ELM327 even prvides more protocols, including CAN.

FAQ: OBD2 Codes Explained.

OBD-II codes consist of a number of parts. Here is a sample OBD2 code:

P0171

Here is a breakdown of what each digit of the code means:

First Character - System

The first character identifies identifies the system related to the trouble code.

• P = Powertrain
• B = Body
• C = Chassis
• U = Undefined

Second Digit - Code Type

The second digit identifies whether the code is a generic code (same on all OBD-II equpped vehicles), or a manufacturer specific code.

• 0 = Generic (this is the digit zero -- not the letter "O")
• 1 = Enhanced (manufacturer specific)

Third Digit - Sub-System

The third digit denotes the type of sub-system that pertains to the code

• 1 = Emission Management (Fuel or Air)
• 2 = Injector Circuit (Fuel or Air)
• 3 = Ignition or Misfire
• 4 = Emission Control
• 5 = Vehicle Speed & Idle Control
• 6 = Computer & Output Circuit
• 7 = Transmission
• 8 = Transmission
• 9 = SAE Reserved
• 0 = SAE Reserved

Fourth and Fifth Digits

These digits, along with the others, are variable, and relate to a particular problem. For example,a P0171 code means P0171 - System Too Lean (Bank 1). To lookup your particular code, visit our OBD Trouble Codes section.

Isuzu Specific Trouble Codes.

3-Pin to 12-Pin for IZUSU

Isuzu/Delco 3-Pin ALDL Adapter Part Number: 02001325 Price: $52.00 USD

Required for diagnosis of 1987 and newer Isuzu engines with Delco Electronics ECMs and 3-pin DLC. Used with the MTS 3100 Mastertech and Tech 1A with the GM 12-pin adapter (02001384).

Isuzu/Hitachi 3-Pin ALDL Adapter Part Number: 02001326 Price: $52.00 USD

Required for diagnosis of Isuzu vehicles equipped with Hitachi ECMs and 3-pin DLC. Used with the MTS 3100 Mastertech and Tech 1A with the GM 12-pin adapter (02001384).

Link : boschdiagnostics.com/parts/diagnostics/mts3100/Pages/MTS3100parts.aspx