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OBD: all you need to know

ByFields Corrielus 2019-08-06 665

OBD (On Board Diagnostic) is an anti-pollution protocol used in the United States; it is an on-board pollution monitoring system integrated into the engine management system; it is standardized and common to all brands; its derivative in Europe is the EOBD (European On Board Diagnostic).


History of OBD

The OBD presents several key dates:

● 1991: application of OBD I in the USA for petrol vehicles;

● 1996: application of OBD II in the USA for petrol vehicles;

● 2001: application of the EOBD in Europe to 2001 series production petrol vehicles, with the Euro III standard;

● 2004: Application of the EOBD in Europe to 2004 series diesel vehicles.

Features of the OBD

As described above, it is based on the OBD II system of the State of California; it is part of the engine electronics and its function is to monitor the proper functioning of the engine components acting on pollution.

To do this, it carries out controls on the effectiveness of sensors and actuators, on the one hand, and rational controls on defined operating values, in relation to a predetermined threshold (example: "lambda" value, i.e. the ratio between the effective mass of air admitted during combustion and the ideal mass of air (14.8 g air for a 1 g petrol); it is equal to 1 for an ideal petrol engine dosage).

OBD: system composition

A clearance witness

Also called MIL (Malfunction Indicator Light), it is activated by the EOBD system for any failure of the emission control system.

The identifier symbol is yellow in colour and represents an engine block.

A standardized data protocol

It continuously monitors:

● The efficiency of the catalyst or FAP (Particulate Filter A).

● The operation of emissions-related sensors: engine management sensors (TDC sensor, phase sensor, coolant temperature, oxygen sensors or differential pressure sensor (HDI), pressure sensors, flowmeter, pedal position sensors, etc., as well as sensors linked to other systems and interacting with the engine function (example: wheel sensors for vehicle speed, etc.).

● The actuators (these are all the elements controlled by the ECU (they therefore receive an electrical signal from it) for engine management: injectors, EGR valve, turbocharger pressure control, FAP regeneration, pre-chaffing plugs, ignition coil (petrol), fuel pressure regulator, etc.

A detector plug

diagnostic plug is a 16-channel interface socket, which makes it possible to communicate with the various ECUs; it also makes it possible to check the parameters linked to the OBD, to delete the memory of the linked faults (of course after having resolved the fault(s)) and thus to turn off the diagnostic light (MIL) on the dashboard.

It must be located in the passenger compartment.

OBD: light on

The light illumination indicates an emission-related fault; it is conditioned by a very precise protocol linked to driving cycles (t° setting - driving at different speeds and load...).

Minor or fugitive defect

In this case:

● The indicator lights up if the fault persists for 3 driving cycles.

● It switches off after 40 consecutive cycles without fault.

Serious defect that could lead to the destruction of the catalyst or FAP

If the defect is serious:

● The indicator lights up after 3 driving cycles.

● It remains on continuously until the workshop intervention and fault clearance with the EOBD controller or diagnostic device.

● If no intervention is carried out, the operation gradually switches to degraded mode (limited mode - cooling fan on - limitation of ancillary functions (air conditioning...) until starting is prohibited).

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