Introduction

Introduction

An on-board diagnostics system is an integral part of the engine control module ECM, it is used to monitor the integrity and effectiveness of the emission system components.

In order to comply with the latest European legislation relating to vehicle emissions for passenger cars and light-duty trucks, vehicles with petrol engines must be equipped with an on-board diagnostics system from 2000MY. On-board diagnostics for diesel engines is not a statutory requirement until 2003 and so engine management systems and OBD for vehicles with diesel engines will not be covered in depth in this document, although some mention will be made of the type of emissions pertinent to diesel engines.

The fundamental requirement for an OBD system is that in the event that an emissions related component malfunctions, the fault is stored in the ECM’s memory. A malfunction indicator light (MIL) is included in the vehicle’s instrument pack which is used to indicate a warning to the driver that a failure has occurred. The fault code stored in memory can be retrieved using diagnostic equipment such as “Testbook” to determine the nature and status of the fault condition.

The objectives of an on-board diagnostics system is to provide a means for the following:

• Fault detection of components relevant to exhaust emissions (performed as part of the engine

management system)

• Fault storage (in ECM internal memory, including failure conditions)
• Fault display (MIL lamp and/or LCD display)
• Fault retrieval (using a diagnostic tool such as “Testbook”)

The OBD system is not designed to directly limit passenger vehicle emissions itself, but rather to check the integrity and operation of other systems and components on the vehicle. A component or system failure could adversely affect fuel efficiency and /or emissions produced; the OBD ensures that these critical items continue to operate to the required standard throughout the lifetime of the vehicle.

The compulsory introduction of E-OBD is likely to have a big impact in the service aftermarket. There is a general fear that a scenario could occur whereby garages become swamped with vehicles returned by customers because the MIL warning light keeps coming on. In reality, this should not occur if the systems perform as design intended. Essentially, On-board Diagnostic systems assist the diagnosis, repair and maintenance of vehicles to provide benefits to both the vehicle owner and the environment. It will also help the garage mechanic to determine the cause of a particular fault much more quickly and accurately and so result in less time for a faulty vehicle to be off the road for repairs.