What are some things that can cause a catalytic converter to fail?
 

I bought a catalytic converter less than 2 years ago and already its rattling. I have recently replaced the valve seals and the valve cover gasket, and I am going to have the fuel mixture checked when I replace the cat. Is there anything else I should check that might have caused it to fail so quickly? Car is a '90 560SEL. Thanks for any input.

Short trips where it does not get up to temp before the car is shut down again and improper fuel mixture.

Are you sure it's not just the shield that rattles?

Freestyler

Converter failure can be caused by:
1. Constant misfiring ( overloads & overheats ) the converter.
2. Excessive oil consumption.

I have seen total converter meltdown in 50 miles, due to a faulty coilwire ( GM ). ;)

Among other reasons, there is great likelyhood that the replacement cat wasn't a genuine MB part. The MB catalyst is about 10 times better made than any aftermarket version. Based on that alone two years isn't bad.

But as others have said a cat can be damaged in 50 miles of meltdown conditions. Those are such when closed loop fuel control is not strictly maintained.

cat failure
 

some of these aftermarket cats are just plain junk. catco is one i can think of in so. cal. there is nothing like a factory cat for most cars. a lot of cats are replaced because fuel mixture was wrong,too rich, which shut down the cat. fix the mixture problem ,take tailpipe readings and the cat is not working.drive the car 20-100 miles and the cat will come back to life. readings are like a new car. this does not happen much because customer wants instant results, not maybe it will come back.
also they wait till the last minute. smog due yesterday. as soon as an aftermarket cat goes on a car ,there is a good chance it will need a new one next smog check. the nox part of these cats are weak.at least half the time the cat was put on with out fixing the fuel mix problem,so again new cat next time.
sorry bout this long reply. the other responses where also right on. try to save the factory cat.
while dealing with cats, a lot of techs are not aware that obd2 fuel mix MUST go rich to lean at a fast pace, if biased rich or lean the nox part of the cat does not work very well. the mix has to cross over the stoich point for the cat to save then use oxygen for nox reduction. biased o2 sensors make the cat look bad. got carried away here thanks rich

All of you are correct, it is an aftermarket "high flow" cat made by Magnaflow. I am pretty sure my fuel mixture is rich so I will have that checked. Olpos, is it really true that if I fix everything that the cat can come back to life? Thanks for the quick responses.

If it's rattling it isn't likely to resecure itself.

also a bad O2 Sensor will harm the cat convertor

Both cats went on the Range Rover due to the use of regular (87 octane) gas when it requires premium (93). [Before i purchased] I believe it is the same in the MB. When they went on the Rover, it sounded like the engine was rapping real bad.

Why? Did the 87 have lead in it?

hi again
 

if your cat is broken you will have to replace it cause the noise will make you crazy.some cats that are broken will still work enough to pass another smog check. the piece inside may be in two large chunks or may be small enough to get inside the resanator or muffler. this leaves you with more rattles.
when the mixture is correct what is inside your cat will clean up and you will get your best numbers possible.
i have seen a lot of broken cats pass the test. but what about the chunks inside. do you really want them in your muffler?
fix the mixture problem,drive a hundred miles and retest. this will tell you what to do next. good luck
food for thought and maybe a different tread. this timing resistor that is removed for power, what if a resistor that retarded your timing more, instead of less was made and put in before a smog check. that should improve all the tailpipe numbers.
this would be for cars that almost pass smog.
nox,co and hc would go down. will experament with this soon. tail pipe numbers with and without the resistor.
got carried away again rich

one more thing
 

o2 sensors can be working when you check them or be out of range. if its old and can be found at a good price, then change it. some times all the checks you do just don't catch it acting up.
plus a new sensor works a lot faster. don't forget to verify that the new sensor works. even new parts can be dead.
buy the bosch brand . less problems
rich

I had a mechanic raise the car up, he thought it failed because my car is lowered and I scraped on it too much. There was quite a few scartch marks on it, but but I have never hit it hard. I always go over bumps real slow and rarely does it ever bottom out. If this is the case though I will just remove the cat and put it back on when smog time comes around. $350 for installed cat vs 50-100 for installation and removal every 2 years... Depends on how the O2 sensor is. olpus let us know how your experiment goes, sound interesting.

emissions failure 92 300ce
 

i recently failed my emissions test in texas...my check engine light is not on, i can't afford to take it to the dealer to diagnos the problem, here is what i failed:

Low Speed Emissions Result:
HC 825 (standard 220)
CO 8.3 (standard 1.20)

I was told it was either my cat or my o2 sensors...any help please!!!!