2 Air Repair OBDII Review Volume 2 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
criteria 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
communicate via the DLC and most that will not are
either missing B+ on pin 16 and/or ground on pin 5. If the
vehicle 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 informa-tion
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.
Technicians 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
monitored systems relies
on another monitored
system to function, then a
difficulty (DTC) interrupts the sequence. You might see in
this case a DTC and insufficient 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
