EPA's Proposal for MOBILE6
            Effects of On Board Diagnostics (OBD II)
                                
                         March 14, 1997


Abstract

     MOBILE5 did not explicitly address the effects of the
proposed On-Board Diagnostic (OBD) systems on vehicle maintenance
behaviour.  Now that the final rules for these systems are in
place, EPA proposes that MOBILE6 attempt to account for the
benefits of OBD for the newer model years.  Although various OBD
systems have been available on some vehicles among older model
years, EPA proposes that only the newest implementation of OBD
(called OBD II) be accounted for explicitly in MOBILE6.

Summary of the Proposal

     EPA proposes a methodology similar to the method currently
used by California in their highway vehicle emission model.  In
this methodology, the effect of OBD is to reduce the number of
high emitting vehicles in the distribution of emissions for model
years subject to the OBD II regulations.  The benefit of OBD
would depend on the definition of high emitters, their occurance
in the fleet and the assumed effectiveness of OBD in eliminating
high emitters.  Both exhaust and non-exhaust (evaporative)
emissions would be reduced.

Supporting Data

     Since OBD II systems are only now being installed on
vehicles and the number of vehicles which would benefit from OBD
systems is low for new vehicles, EPA is not aware of any
available vehicle data that would apply directly to the issue of
benefits for OBD in MOBILE6.

Methodology

     o The method assumes that nearly all high emitters will be
     detected by the OBD system and reported to the vehicle owner
     via a dashboard light.  

     o The method assumes that during the warranty period, when
     repairs are "free" to the vehicle owner, that the vehicle
     owner will have all emission related problems repaired
     promptly.  

     o After the warranty period, vehicle maintenance behaviour
     would revert to "normal" maintenance, similar to what would
     be expected without OBD systems.  

     o Inspection programs which utilize the OBD system to
     identify vehicles could be assumed to identify nearly all
     high emitting vehicles.

Outstanding Issues

     o The definintion of high emitters is being addressed
     separately from OBD effects.

     o Depending on the definition of high emitters, OBD systems
     may not identify all high emitters.

     o There may be some delay in getting repairs.  This would
     reduce the overall effectiveness of OBD by increasing the
     number of high emitters in the fleet at any given time, even
     if all high emitters can be identified by OBD.  This delay
     factor, the time between the occurance of a high emitter and
     the identification and repair of the vehicle, would also
     reduce the effectiveness of inspection programs, depending
     on the frequency of inspection.

     o There is an interaction between the number of high
     emitters and the effects of corrections to the basic
     emission rates.  As a result, adjustments to the emission
     rates for OBD equipped model years may require adjustments
     to the corresponding correction factors.