AIR REPAIR OBDII REVIEW 2008 13
Catalytic Converters and OBDII
From the September 2005 issue of Air Repair.
By George Generke, Automotive Instructor,
College of Du Page
In Illinois a vehicle that has failed an OBDII Emission Test
with any catalyst DTC (P0420-P0439) will not pass on a retest
until the Catalyst Efficiency Monitor has run to completion and
passed without setting another catalyst DTC. Some technicians
simply replace the CAT because of a DTC and then suffer the
consequences of an unhappy customer returning with an MIL
illuminated as a result of a recurring DTC, and worse yet a
repeat emission failure after they just “fixed it”. The technician
today needs to reference warm up cycles, drive cycles, trips,
monitors, and enable criteria in order to understand why the DTC
The two common methods used for determining catalyst
efficiency for an IM240 emission failure are oxygen storage
capacity and HC conversion to CO2, and both of these tests use
a pass/fail threshold of 80 percent. These tests are inadequate for
an OBDII vehicle! The OBDII catalyst must work at much higher
efficiencies, often above 95 percent. The method we use to test
the catalyst really isn’t as important as getting the CAT to pass
the catalyst monitor built into the PCM diagnostic programming.
There are many pitfalls to this, and one is taking it on blind faith
that the PCM can accurately determine CAT efficiency every time.
Catalyst monitoring is normally done under either idle or
light-load, steady-cruise conditions. The catalytic converter
conversion efficiency for HC and CO can be greatly reduced
by an AFR running as little as three to five percent rich. A lazy
O2 sensor that is marginal at best when tested, yet is still within
the pass criteria for OBDII, can cause the actual AFR to be a
little richer than 14.7:1, or stoichiometry (Lambda for short).
There have been numerous case studies done where a P0420/
P0430 DTC was “fixed” by replacing one or more O2 sensors,
or “caused” by replacing a post-CAT O2 sensor only. The MAF
sensor will usually underestimate the air coming into the engine
when it becomes contaminated, resulting in positive fuel trim
values. In addition to the MAF, the TPS will read a value that
the PCM may compare to the incorrect MAF value and “think”
the engine is under a heavier load, putting the fuel delivery
calculation into a different fuel trim cell. If the CAT monitor will run
in this fuel trim cell, the monitor will likely fail the CAT due to the
additional fuel loading (running rich). Or the CAT monitor may not
even run at all due to this condition.
Catalytic converter contamination is another issue facing the
P0420/P0430 DTC. The current CAT killer is sulfur, and poor fuel
grades (or maybe I should say fuel brands) will often have higher
amounts of sulfur than others. It may be possible to reverse
the condition of sulfur contamination with a change in fuel to a
low sulfur grade, and then a variety of engine load and run time
conditions. How long and how hard do you drive it? That’s really
dependent upon how bad the contamination is. Extreme engine
loading conditions such as pulling heavy trailers for prolonged
periods can raise catalyst temperatures to damaging levels. Don’t
forget, you already know engine misfires are a serious threat
to catalysts. Mode $06 can give you solid data on the success
of your repairs after running a monitor under the enable criteria
conditions, but Mode $06 is not available for all vehicles, and
interpreting Mode $06 can be a frustrating experience at first.
Do not overlook the possibility that someone has been there
before you and did the unthinkable, such as installing a “used”
PCM from a salvage yard or some other source of used parts.
The emission calibration codes are very vehicle specific, and
customers have replaced PCMs on their own to save money on
repairs, not understanding that they may have actually caused
the problem you are faced with now.
The problem with not using an OEM catalyst is the CAT
is most likely not certified to meet the EPA requirements for
OBDII, and it is less likely to meet the specific requirements to
pass the CAT monitor on the particular vehicle it will be installed
on. There are very few suppliers of aftermarket CATs that offer
OBDII applications and even those that do only offer a very small
selection at this time. Make sure the catalytic converter you install
on an OBDII vehicle is “specific” by catalog application to the
vehicle it is going on.
From the October 2004 issue of Air Repair.
By Michael Hills, Engineer, Technical Services;
Division of Mobile Source Programs, Illinois EPA
If the MIL is commanded on for a transmission code, it is emissions
related. OBDII is required to monitor all powertrain components
that effect emissions, or provide diagnostic input, or receive
commands from the PCM. The transmission controls the amount
of power going from the engine to the wheels. If the transmission
is not working properly, the efficiency of the power transfer
will be degraded. Simply stated, the engine of a vehicle with a
malfunctioning transmission will have to work harder to provide the
same amount of vehicle speed. A harder working engine will require
more fuel which will result in higher emissions. If the sensors
that monitor the transmission are not functioning properly, the
PCM cannot determine if the transmission is working properly,
resulting in a “Command On” status, illuminating the MIL. The
decision to include these sensors is made by the manufacturers.
Transmission codes can also indicate problems with
engine misfire. Most vehicles detect misfire using a crankshaft
position sensor to detect even a minute fluctuation in crankshaft
acceleration and rotation. Shifting on rough roads can cause
false readings. Therefore, some manufacturers unlock the
torque-converter clutch when strong road vibration is detected. If
the transmission is not functioning properly, the unlocking of the
torque-converter might be triggered prematurely, resulting in an
engine misfire that could go undetected.
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