I have a '98 E320 Wagon with about 86,000 miles. The check engine light came on last month, so I took it to my mechanic and the trouble code was P0422 (catalytic converter). He reset the code and all was well for about a week and the light came on again. The same code came up, he reset it again and it lasted a few days this time. After a few days of the light being on, it went out by itself. About a week later, the light came on again.

Given the high price (PartsShop MUCH less expensive than dealer) of a replacement converter I would like to make sure it is really the problem before I replace it. If I really do need to replace it, the mechanic has two conflicting code documents: one says driver's side converter, the other says cylinder bank one, which could be the passenger side.

Any suggestions or help would be appreciated!

PO422 is for the left side cat.

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Mercedes Benz Master Tech - Retired

Thanks. The light is now off again. Any possibility this could be something other than the converter?

The CEL light went out for quite a while. It reappeared a few months ago...I bought a code reader....It had two codes....P0170 and P0173. From other posts on the forum, I figured it was the mass air sensor. I ordered the mass air sensor from MercedesShop last week and installed it yesterday. No problem until just now. The CEL came back on and it now is showing #1P0422, #2P0170 and #3P0173.....Help!

What are your adaptation numbers?

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

Steve:

I have done a search on adaptation numbers on the forum and read all of the related posts...I THINK I understand......!

My code reader is an Innova Equus 3100....I don't believe this reader is capable of giving me the adaptation numbers......Is it now time to take it to the mechanic?

Thanks!

I had the driver's side catalytic converter replaced yesterday and CEL reset.....CEL came back on today with code P0422....any suggestions?

Factory cat or aftermarket?

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Todd Haven
Parts Manager
MPH Automotive
Houston,TX

Aftermarket....

We have tried a few suppliers of aftermarket cats, and
will do so no longer on OBD-2 cars.

The code is actually a "catalytic convertor efficiency" discrepency.

This parameter is set by the factory, and cannot be altered.
We have seen a large number of 210 cars with failed cats, some 202's.
BMW 740's seem prone, as well. Very often the substrate in the cat breaks up, and rattles annoyingly at idle.

The aftermarket cats we tried were replaced with factory units, and the
problem was cured. Lesson learned.

Pre-96 cars we have used the aftermarket cats with success.
(1 O2 sensor per bank, vs. 2 per bank in OBD-2 cars).
Less stringent monitoring.
These are not the generic, cut-the-pipe-and-weld-it-in type cats.
We use those only on 80's Euro stuff where there is no real option.

We have emissions testing, and the annual state inspection can not be performed on a car with check engine light on, or readiness tests not having been met.

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Todd Haven
Parts Manager
MPH Automotive
Houston,TX

Thanks, I guess I will live with it...

What about the possibilty that the problem is actually one or both of the rear O2 sensors (post-cat diagnostic probe)?
Gilly

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Since the car had been checked by a tech, I presumed the sensor voltages had been checked. Note to self, "stop leaping to conclusions".

In our experience, the cat convertor efficiency codes mean convertor failure virtually every time, but o2 sensor voltages should be checked first, of course.

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Todd Haven
Parts Manager
MPH Automotive
Houston,TX

Just a thought anyways, I know there are codes which will fault the rear O2 sensors, but I don't recall any that will tell you "slow response time", etc like the front ones, as the rear ones fluctuate very, very little since they are behind the cat. So, leads me to wonder, will the diagnostic chain lead to a code for a bad cat, when in fact it's a "dirty", ie contaminated or "lazy" rear O2 sensor? I mean "I don't know", but worth considering. I would imagine somewhere it must be mentioned in the MB diagnostic system for that code, such as in the diagnostic manual, I don't have access to those anymore, so can't check, but I remember you'd look up the code and almost always it'd list at least 3 or 4 different possiblities for a code and where to go to check out each possibility.

Gilly

PS and since I'm on a roll, I'd like to mention that diagnosing problems like this is never just a matter of reading a code and replacing a part. It's NOT that easy, the codes can be caused by a bunch of different things. Very seldom will will you read a code, check out what the code means, and the manufacturer will simply tell you to replace a part. There can be several different faults that can set the same trouble code, it's up to the mechanic to determine what the real problem is. Anyone who doesn't realize this is not a mechanic but simply a "parts replacer". This is NOT aimed at anyone reading this, just to clarify that......just stating the obvious.

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Agreed, how many EGR valves do you think have been sold for 104,112 and 113 motors because the big,box store checked codes for free,
and the "computer" said EGR malfunction--
See that code 4-5 times a week,
sell maybe 1 valve a year.

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Todd Haven
Parts Manager
MPH Automotive
Houston,TX