OBD-ii

Another limitation of OBD-I was that it couldn’t detect certain kinds of problems such as a nonfunctioning or missing catalytic converter, ignition misfires, or evaporative emission problems. In addition, the MIL lamp would only illuminate after a failure had occurred, it had no way of monitoring progressive deterioration of emissions-related components.

OBD-II

CARB proposed a new set of standards for an enhanced OBD system in 1989 which were incorporated into the federal Clean Air Act of 1990. In 1994, the US Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) issued this strict new set of guidelines and a phase-in program concerning the application of on-board diagnostics systems began; this was to be mandatory for certain passenger vehicles. These guidelines known as OBD- II were designed to detect emissions systems related malfunctions and facilitate their repair before vehicle performance could deteriorate.

The MIL warning light on OBD-II 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 OBD-II 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 OBD-II over OBD-I 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 OBD-II regulations were expanded to apply to all passenger cars in the US market (i.e. since 1st January 1996 all cars built for the US market have OBD-II systems).

Although some manufacturers started incorporating OBD-II in various models as early as 1994, some of the earlier OBD-II cars were not 100% compliant.