According to the 2005/55/EC legislation:
On-board diagnostic systems
2. From 1 October 2008 for new type-approvals and from 1
October 2009 for all type-approvals, a compression-ignition
or
a gas engine type-approved by reference to the emission limit
values set out in row B2 or row C of the tables in Section
6.2.1
of Annex I, or a vehicle propelled by such an engine shall be
fitted with an OBD system that signals the presence of a fault
to
the driver if the OBD threshold limits set out in row B2 or
row
C of the table in paragraph 3 are exceeded.
The OBD system shall also include an interface between the
engine electronic control unit (EECU) and any other engine or
vehicle electrical or electronic systems that provide an input
to
or receive an output from the EECU and which affect the
correct functioning of the emission control system, such as
the
interface between the EECU and a transmission electronic
control unit.
3. The OBD threshold limits shall be as follows:
Compression-ignition engines
Mass of oxides of nitrogen (NOx) g/kWh
Mass of particulate (PT) g/kWh
B1 (2005) 7,0 0,1
B2 (2008) 7,0 0,1
C (EEV) 7,0 0,1
Emission values for reference: http://minibusgrp.com/?page_id=438
Basically the thresholds for OBDs on buses and trucks conforming
to the legislation will have it set about Euro 1 standard. Newly
manufactured vehicles will need to meet the current standards (Euro
5 in force October 2009) but they will face slow degrading of
emission. All vehicles would face this problem too.
