OBD-II Introduction

What is OBD-II

OBD-II standard is an improvement of OBD-I. It features better performance in both capability and standardization. On the base of ALDL and OBD-I, the OBD-II specifies the type of diagnostic connector and its pinout, adds the electrical signaling protocols and the messaging format, and most importantly, provides an extensible list of DTCs, making it easier for mechanics to work and communicate.
On OBD-II cars, there is usually an OBD-II DLC under the dashboard on the driver’s side. In the port, there lies a pin in the connector that provides power for the scan tool from the vehicle battery, which avoids the need to connect a scan tool to another power source, although some technicians still connect the scanner to another power source to protect data in the car’s computer in case of a loss of electrical power due to a malfunction.
The invention of this standardization makes it possible that a single device can read the on-board computer(s) in any vehicle. Despite the fact that only emission-related codes and data are required to be transmitted through the system at that time, most manufacturers have made the OBD-II DLC the only one in the vehicle through which all systems are diagnosed and programmed. These trouble codes are 4-digit. Each character indicating to problem in different parts.
The first character, such as P, B, C and U, troubleshoot the system related to the error code:
P means Power train
B means Body
C means Chassis
U means Undefined
The second digit tells you whether it is a generic code or a manufacturer specific code:
0 means Generic
1 means Manufacturer specific
The third digit identifies the type of sub-system related to the code:
1 means Emission Management (Fuel or Air)
2 means Injector Circuit (Fuel or Air)
3 means Ignition or Misfire
4 means Emission Control
5 means Vehicle Speed & Idle Control
6 means Computer & Output Circuit
7 means Transmission
8 means Transmission
9 means SAE Reserved
0 means SAE Reserved