OBD History (page 2 of 2)

The MIL warning light on OBDII systems is set to illuminate any time a vehicle’s hydrocarbon (HC), carbon monoxide (CO), oxides of nitrogen (NO_x) or evaporative emissions exceed 1.5 times the Federal Test Procedure (FTP) standards for that model year of vehicle. This includes:

• Any time random misfires cause an overall rise in HC emissions
• Any time the operating efficiency of the catalytic converter drops below a certain threshold
• Any time the system detects air leakage in the sealed fuel system
• Any time a key sensor or other emission control related device fails
• For diesel systems – any time a fault in the EGR system causes NO_x emissions to rise

Therefore the MIL light may illuminate even though the vehicle seems to be running normally and there are no apparent driveability problems. Because a vehicle may appear to be running well even though the emission levels have increased due to a system or component fault, the MIL warning lamp provides a means for alerting the driver of the vehicle that they are causing pollution and they need to get their emissions problems fixed.

Because motorists may ignore the warning lamp even when it is indicating a fault, regulators want to incorporate OBDII into existing and enhanced vehicle emissions inspection programs. If the MIL lamp is found to be on when a vehicle is tested, it will not pass the tests even if the exhaust pipe emissions are within acceptable limits.

Another important development of OBDII over OBDI was the introduction of defined standards for trouble codes and diagnostic equipment. The Air Resources Board required that all manufacturer's must conform to standards for the following:

• 16–pin serial data link connector with specific pins assigned specific functions
• electronic protocols
• diagnostic trouble codes (DTCs)
• terminology

In 1996, the phase-in period for vehicles in California was to be completed and the scope of the OBDII regulations were expanded to apply to all passenger cars in the US market.

In addition to more advanced software, OBDII systems typically include the following features over and above that used on OBDI systems:

• Twice the number of oxygen sensors than non-OBDII 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