OBD-ii (page 2 of 3)

In addition to more advanced software, OBD-II systems typically include the following features over and above that used on OBD-I systems:

• Twice the number of oxygen sensors than non-OBD-II vehicles, with the sensors usually

being heated (HO₂S). The additional HO₂ sensors are positioned downstream of the catalytic

converters to determine catalyst efficiency.

• More powerful electronic control modules (ECMs)
• Electrically Erasable Programmable Read Only Memory (EEPROM) which allows the ECM to

be reprogrammed with the latest software changes using external computers connected via

the diagnostic connector.

• Modified evaporative emission control systems with a diagnostic switch for purge testing or

an advanced EVAP system with vent solenoid, fuel tank pressure sensor and diagnostic test

routine

• EGR systems with linear EGR valve which is electronically operated and has a pintle position

sensor

• Sequential fuel injection rather than multiport or throttle body injection
• Manifold Absolute Pressure (MAP) sensor and Mass Air Flow (MAF) sensors for monitoring

engine load and airflow

Standardization is an important part of the OBD regulations and it facilitates for access to emission-related fault codes, emission-related powertrain test information (i.e., parameter values), emission related diagnostic procedures, and stored freeze frame data based on industry specifications.

Standardization of the message content (including test modes and test messages) as well as standardization of the downloading protocol for fault codes, parameter values and their units, and freeze frame data are specified in SAE recommended practices “OBD II Scan Tool” (J1978) which was issued in June 1994, and “E/E Diagnostic Test Modes” (J1979) which was issued in July 1996. Fault codes, parameter values, and freeze frame data have to be capable of being downloaded to a generic scan tool which meets these SAE specifications.

Despite the move towards standardisation, there are three basic OBD-II protocols in use in the US market. Each of the protocols has minor variations on the communication pattern between the onboard diagnostic computer and the scan tool. Chrysler products, all European models and most Asian imports use ISO 9141 circuitry, General Motor cars and light trucks use SAE J1850 VPW (Variable Pulse Width Modulation) and Fords use SAE J1850 PWM (Pulse Width Modulation) communication patterns. These diffrences are reflected in the pin numbers used for diagnostic communication on the diagnostic connector. Even though there are three OBD-II electrical connection protocols, the command set is fixed according to the SAE J1979 standard.

Other SAE standards relating to OBD-II include:

• J1930 – common acronym terms for all critical Emission Control components
• J1962 – common Data Link Connector (DLC), and its location
• J2190 – common Diagnostic Test Modes
• J2012 – common Diagnostic Trouble Codes