Air Repair OBDII Review Volume 3 Number 1
What Do We Know About OBDII Now?
From the January 2005 issue of Air Repair.
By Al Santini
Illinois has been testing OBDII systems with pass fail cri-teria
for about a year now, and we know much more about
how the system works or does not work than we knew in
January 2004. But first a quick review...
A vehicle that is presented for an OBDII test will first
receive a gas cap test and then be scanned through the
DLC. The vast majority of the vehicles will in fact com-municate
via the DLC and most that will not are either
missing B+ on pin 16 and/or ground on pin 5. If the ve-hicle
will communicate via the DLC, a check of applicable
readiness monitors will occur. If the correct number of
monitors have been run, MIL function is analyzed and the
vehicle passes or fails. This abbreviated explanation gives
an overview of the test. More information is available in
the Outreach OBDII seminars.
So what do we know about the test after months of
testing thousands of vehicles? The first item of interest is
the system is doing exactly what it was designed to do.
Remember that OBDII is an emission testing strategy that
includes most of the engine management functions that
were present in OBDI. The design of OBDII is supposed
to allow independent testing of emission functions with
the ability to identify failures that might allow the vehicle
to produce in excess of 150 percent of the Federal Test
Procedure. Without a doubt it is doing exactly this. Techni-cians
that attend the OBDII seminars indicate that vehicles
that have the MIL illuminated or commanded on do have
something wrong and can be fixed to a level that will allow
the MIL to be commanded off by the PCM. There has been
virtually no false MILs on vehicles. If the MIL is on - there
are fixable problems. Once they are fixed the MIL will be
commanded off. The system is doing exactly what it was
supposed to do.
However, readiness or lack of monitors run remains
an issue. Currently 16 percent of the vehicles are rejected
because of insufficient monitors run. These vehicles fall
into one of three different categories:
Category One - A couple of monitors remain unset.
Frequently these vehicles require some specific driving
conditions to get the monitors to run. The customer just
does not drive the vehicle in the “correct” manner. An
example of this is a person who does not drive the vehicle
at highway speeds. It is likely that the Catalytic Converter
monitor might not run. The technician will be required to
drive in a very specific manner paying attention to speeds
and loads that will allow the monitor to run. Note: Don’t
forget running a monitor is what allows the system to
generate a DTC. No monitor = no diagnostic trouble code
generated. The CD that is given out at the OBD seminars
helps greatly by identifying the driving conditions required
for monitor completion.
Category Two - A specific monitor or group of
monitors will not run. Many times this situation occurs
because the system has determined that there is a problem
in something that is used in the diagnostic procedure. For
example, a bad O2S heater will prevent the O2S monitor
from running. If the O2S
monitor does not run, then
the CAT monitor will not
run. If one of the moni-tored
systems relies on
another monitored system
to function, then a diffi-culty
(DTC) interrupts the
sequence. You might see in this case a DTC and insuffi-cient
monitors run. Repair the vehicle and after the DTC is
removed the sequenced monitors now run. The illustration
shows a vehicle with a bad O2S heater. The heater monitor
has run, generated a DTC and shut down everything that
uses the O2S as an input.
Category Three - No monitors run. This may sound
bizarre but does happen every so often. It appears that if
certain components are slightly beyond normal criteria, the
PCM might prevent the monitors from running until the
component is replaced. We have seen examples of O2S’s
preventing all monitors from running and yet the vehicle
has no O2S DTC’s. Should this happen? Probably not, but
oh well....
If everything worked exactly as it should 100 percent
of the time, probably none of us would be needed. Many
times these vehicles need to be approached as if they were
an OBDI system. Scan them and use your DSO to identify
suspect sensors. It is also a possibility that they will need
to be reflashed.
Don’t let the monitor
issue cloud your thinking.
Something is preventing
them from running. Try to
drive the required trace. If
that does not work, start
looking for something that
is preventing the system from running the monitors. The
system will work as designed and identify problems once
the monitors have run.
CODES DESCRIPTION STATUS
CAT Catalyst Efficiency Status: NOT READY
CATHEAT Catalyst Heating System Status: Not Supported
EVAP Evaporative System Status: Not Supported
AIR Secondary Air System Status: Not Supported
AC Air Conditioning Refrigerant Status: Not Supported
O2S Oxygen Sensor System Status: NOT READY
O2SHEAT Heated Oxygen Sensor System Status: Ready
EGR Exhaust Gas Recirculation System Status: NOT READY
