Catalytic converters not always the problem with catalyst efficiency - P0420 and P0430 are juts two generic codes related to catalyst efficiency. - Motor Age - Automotive training, certification & par
(LEFT) Converters can be damaged by overheating. A quick rap on the can rattled the substrate on this GM converter. Make sure
you repair the cause of the failure and not just the symptom. (INSET) The graph on the left shows the oxygen sensor patterns
before and after a healthy cat. Notice the lower pattern on the right side graph. This is the post-cat sensor signal on a
cat that has lost oxygen storage capacity and now mirrors the pre-cat signal.
The catalytic converter is the emissions system's last chance to clean up exhaust gases before they reach the atmosphere.
It does so by storing excess oxygen during times of lean combustion mixtures and releasing oxygen during rich mixtures, oxidizing
the combustion gases and converting them to gases a little more friendly to the environment. But the catalytic converter can
only do so much, and it is the job of the Engine Control Module (ECM) to not only feed it the correct gases to begin with,
but to also monitor its health.
Notice the delay between the pre-cat sensors reaction to the fuel shift and the post-cat sensors reaction to the same shift.
This is a direct relationship to oxygen storage capacity.
There are two types of failures associated with catalytic converters: thermal and chemical. Thermal damage is a result of
overheating the converter. This overheating melts the substrate and leads to broken pieces of the substrate rattling around
in the converter housing and/or restrictions to exhaust flow and higher backpressure in the exhaust system. Chemical failures
generally are a function of age, as the substrate material is used in the oxidation process. Both impact the ability of the
catalytic converter to store the oxygen it needs to do its job. It is this storage capacity that is tested by the ECM when
deciding whether the cat is still capable of maintaining emissions where they should be.
When oxygen storage capacity is low, the post-cat sensor reacts almost at the same time the pre-cat sensor does.
The ECM's testing falls under the heading of the Catalytic Converter Monitor. This monitor is a non-continuous monitor, running
only once for any given key on-key off drive cycle as long as all the conditions needed to run the tests incorporated in the
monitor are met. These conditions vary by manufacturer to manufacturer and year. Most catalytic converter codes are "2-trip"
codes, meaning that, in the event of a detected failure, the ECM must see the same failure on the next consecutive monitor.
Regardless of the specifics, they all rely on information from both the pre-cat oxygen sensor and the post-cat oxygen sensor.
Pete Meier is an ASE certified Master Technician and sponsoring member of iATN. He has over 35 years practical experience as a technician and educator, covering a wide variety of makes and models. Now, as technical editor for Motor Age, his primary goal is to bring working technicians the information they need to succeed.
Articles by Peter F. Meier
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grease monkey / temple, GA, UNITED STATES
Posted 2009-06-01 08:46:50.0
I have had 2 cats that stopped up the exhaust muffler, I work on bigger trucks ( Med Duty) and parts are expesive. If the cat is empty, the muffler is FULL !!!
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