My car failed the air care test. mechanic
says it must be the cat is gone.
Does anyone know how to test, if the cat
is plugged or not? Would not mind changing it
if it toast but would be a stinker if it aint
the culprit.
Also, the cost for an OEM and aftermarket?
Thank you all.

There are many ways to test the catalyst, but they depend on what you suspect the cat is doing wrong. They all require either tooling or experience to evaluate properly.

A simple test to see if a cat is plugged can be done with a vacuum gauge. Much is subjective but basically the vacuum at idle should go up when the engine is ran at higher rpm UNLOADED. In other words if you raise the rpms to 2000 from idle speed the vacuum should be greater. If the cat is plugged the vacuum will be less.

The problem is that a cat looses its effectiveness long before it plugs. The degree of restriction causes performance problems and the worse it is the easier to find using the above method.

The ability to actually clean the exhaust can be tested by an exhaust gas analyzer and is what most emissions testing does. If you read the gasses before and after the cat the true efficiency is gained.

A simpler less exact method for gauging the cats effectiveness would be with an infra red temp probe (like used for tires). The cat should be hotter at the exhaust end than the intake end while the engine is running (with the cat fully heated).

The factory cats are enormously expensive (probably over $1500), aftermarket are usually less than $500 and are probably not a great deal as their abilities (both immediately and down the road) are worse than 3 to 1. They probably weigh less than a third and are obvoiusly less shielded and smaller in stature. But they can get a car passed. Its a shame that used cats are illegal as I would much prefer a used MB cat to any aftermarket I have seen.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

thanks steve for your prompt reply. is it true
that running rich (fuel mix) will contribute to
damaging your cat?

i've owned several cars, mostly Japanese cars,
but never had replaced any cats.

At a recent seminar, the instructor told of a rental car that he destroyed the cat in a hundred miles drive. I can't remember what was wrong with the auto but the cat died because it got overheated from too much fuel.

The modern car is designed to put the exact amounts of fuel and air into the cat and it makes the continued conversion to water, carbon dioxide, and heat. Anything that causes the proportion of fuel to rise causes overheating in the cat. The internal structure of the cat is basically a ceramic and will take more heat than most metals but as it is right in the fire they melt into a ball of glass fairly quickly. This is the reason for the strict misfire counter rules in OBDII. The quickest way to get too much fuel is to have a misfire where the whole charge of fuel winds up in the exhaust (such as a plug wire arcing). You can immagine the energy the cat must dispense with.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician