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Discussion Starter #1
Hope everyone is doing well and the card even better.

TLDR
99 S500 (built in 98 I think because of rectangular headlight shape, a little over 125k miles) help with exhaust. Do I need all 4 cats or can I just keep 2 if so what is situation with O2 sensors.

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Little about me. Broke law school student for the time being.
Previously fixed up and drove 2 97 E320s 1 98 E320 1 96 E300 D (all of the Es were cocaine white, tan interior and over 250k miles and drove strong till we got rid of them) and a 99 328i convertible. Dad has said we're a Mercedes family and after he owned many old ones, he bought a new 2013 ML350 which seized up soon after in 3in of water during the Memorial Day flood in Texas. That didn't make much sense to anyone, but we decided you just can't beat an old Benz. On the BMW I was in my " I want something fast" phase and replaced the intake manifold with one from an older model which let in a lot more air and replaced the cams with some I got from an M3, never tuned it, cat back was deleted, and needless to say that was the 3rd engine I destroyed in the BMW and decided it still wasn't as much fun, relaxing, classy and reliable as any of the w210s.
I say all that to infer that I have some sort of idea about working on cars, though not as much as I'd like. But I would like to learn from the veterans here.

While in school I haven't been able to give as much attention to a project sitting at home, but as winter break comes after finals I'm excited to get my nails dirty.

Got a 1999 S500 sitting in the garage we got from a family friend whose wife used it as her daily till she passed away with cancer. The car hadn't been started for years but we got it running, brought it home, and it drove fine for a couple days then just died out. Wasn't a battery issue and mechanics let us around in circles changing a lot. After replacing plugs, MAF, ignition coils, fuel rail and more it turned out to be a fuel pump which I suspected from reviewing forums with similar sort of issues, but the mechanic said it could never be.
Took it home replaced it and boom worked.
Then while I was in school driving pop's Camry since I sold my E300D 2 days prior to class starting, I got a call from dad that the secondary cat on the driver side was red as hell and the car was stalling out. Turned out to be some sort of fuel actuator sensor or something failing causing the car to run rich from my understanding. Pops found someone to remove the cat and it turned out to be clogged and that's history. Unfortunately one of the most annoying CELs I've encountered in any Mercedes showed up, O2 sensor failure.

Now here is my issue. While I haven't been able to find great diagrams to help me understand what I'm looking for, and I've been away from the car due to school, is it the case that each exhaust pipe stemming from the headers has 2 cats for a total of 4?
If 4, can I just replace the piping from the headers and just have an O2 sensor installed on each pipe with a single cat on each (2) instead of 4?
I thought the O2 sensor is located right before the cat so I'm not sure why removing a secondary cat would affect the O2 sensor?
I'm also concerned but can't speak to the condition of the primary cats that have a wired shape and I only ask to replace the whole thing because I haven't been able to find any with the strange inlet outlet kidney shaped as the stock ones from pics I've come across.

Essentially here is my bad depiction of what I'm asking:
Before:
Headers (H)==O2=©1====O2=©2==Res==Muf.
After:
(H)=======O2©1=====Res====Muf.

I understand that's not too enlightening as I can't figure out the Bank#/Sensor# situation but I'd like to either replace what was removed with a $45 aftermarket cat or just have 1 set. If anyone can guide me how to go about this I'd appreciate it, I have my obd2 scanner on the way so hopefully it'll clear it up but I just want the car to run without issue since it is a magnificent machine. Can I drive it as is without the smog test concern?
Can I just replace the O2 sensor that comes up first, clear, and see if it comes back and it was just an O2 issue not a cat related one?

Sorry for rant. I am obliged. Have a great holiday season!
 

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Premium Member
'99 S420 116K
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1,380 Posts
Personally, I'd keep the exhaust system as stock as possible. I think the computer monitors and evaluates each O2 sensor to determine optimal fuel settings and malfunctions in the CATS. Sure, you can play with the entire system and probably gain horsepower, noise, whatever, but the original design works well enough.
 

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'97 CL600
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2,854 Posts
I have a '97 CL600. Lost a cat in the remotest part of our desert, no cell service, YIKES! Anyway I finally managed a tow back to my mechanic. I have two cats and 4 O2 sensors. One cat was bad. I replaced BOTH of them with Magnaflows (CA approved) for $250 each. You (and I) have an O2 sensor in front and one after each cat. I understand they are BOTH needed to input the computer as to the efficiency of the cat. And I would suggest a shop (not a dealer) that can run a Star C computer scan on your car. Don't piecemeal it. You said a $45 cat??? An OEM replacement is about $1200 EACH and a good Bosch O2 sensor is about $60-90 depending on who is selling them. Check out Magnaflow and see what they recommend for Texas. Do you have "smog" checks like they do in Kaliforniastan? If so, you will need cats that are legal in the State, not cheap $45 ones!
Anziani
 

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Discussion Starter #4
Thank you all for the responses. Can I get 1 type of cat and replace all 4? I'm just not sure if the primaries are a different shape or whatever. Like even for this, it says on Amazon it'll fit the front right but why wouldn't it fit front left and the secondaries? And if I do get something it'll be like this eh, with the slot for the o2s? Thank you for feedback. As a broke student, oem is out if the question! And I can run non-carb compliant in Texas. I'd assume those are more open flow cats so I'd prefer that. I read Walker makes great ones for the Benz too but I just don't want to buy cats and turns out they won't even fit.
2612811
 

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Hope everyone is doing well and the card even better.

TLDR
99 S500 (built in 98 I think because of rectangular headlight shape, a little over 125k miles) help with exhaust. Do I need all 4 cats or can I just keep 2 if so what is situation with O2 sensors.

-------------------
Little about me. Broke law school student for the time being.
Previously fixed up and drove 2 97 E320s 1 98 E320 1 96 E300 D (all of the Es were cocaine white, tan interior and over 250k miles and drove strong till we got rid of them) and a 99 328i convertible. Dad has said we're a Mercedes family and after he owned many old ones, he bought a new 2013 ML350 which seized up soon after in 3in of water during the Memorial Day flood in Texas. That didn't make much sense to anyone, but we decided you just can't beat an old Benz. On the BMW I was in my " I want something fast" phase and replaced the intake manifold with one from an older model which let in a lot more air and replaced the cams with some I got from an M3, never tuned it, cat back was deleted, and needless to say that was the 3rd engine I destroyed in the BMW and decided it still wasn't as much fun, relaxing, classy and reliable as any of the w210s.
I say all that to infer that I have some sort of idea about working on cars, though not as much as I'd like. But I would like to learn from the veterans here.

While in school I haven't been able to give as much attention to a project sitting at home, but as winter break comes after finals I'm excited to get my nails dirty.

Got a 1999 S500 sitting in the garage we got from a family friend whose wife used it as her daily till she passed away with cancer. The car hadn't been started for years but we got it running, brought it home, and it drove fine for a couple days then just died out. Wasn't a battery issue and mechanics let us around in circles changing a lot. After replacing plugs, MAF, ignition coils, fuel rail and more it turned out to be a fuel pump which I suspected from reviewing forums with similar sort of issues, but the mechanic said it could never be.
Took it home replaced it and boom worked.
Then while I was in school driving pop's Camry since I sold my E300D 2 days prior to class starting, I got a call from dad that the secondary cat on the driver side was red as hell and the car was stalling out. Turned out to be some sort of fuel actuator sensor or something failing causing the car to run rich from my understanding. Pops found someone to remove the cat and it turned out to be clogged and that's history. Unfortunately one of the most annoying CELs I've encountered in any Mercedes showed up, O2 sensor failure.

Now here is my issue. While I haven't been able to find great diagrams to help me understand what I'm looking for, and I've been away from the car due to school, is it the case that each exhaust pipe stemming from the headers has 2 cats for a total of 4?
If 4, can I just replace the piping from the headers and just have an O2 sensor installed on each pipe with a single cat on each (2) instead of 4?
I thought the O2 sensor is located right before the cat so I'm not sure why removing a secondary cat would affect the O2 sensor?
I'm also concerned but can't speak to the condition of the primary cats that have a wired shape and I only ask to replace the whole thing because I haven't been able to find any with the strange inlet outlet kidney shaped as the stock ones from pics I've come across.

Essentially here is my bad depiction of what I'm asking:
Before:
Headers (H)==O2=©1====O2=©2==Res==Muf.
After:
(H)=======O2©1=====Res====Muf.

I understand that's not too enlightening as I can't figure out the Bank#/Sensor# situation but I'd like to either replace what was removed with a $45 aftermarket cat or just have 1 set. If anyone can guide me how to go about this I'd appreciate it, I have my obd2 scanner on the way so hopefully it'll clear it up but I just want the car to run without issue since it is a magnificent machine. Can I drive it as is without the smog test concern?
Can I just replace the O2 sensor that comes up first, clear, and see if it comes back and it was just an O2 issue not a cat related one?

Sorry for rant. I am obliged. Have a great holiday season!
The # 1 o2 is the closest to the motor but dont be surprised if the code doesn’t go away right away. I replaced all of mine and it took about 6 months for the code to stop
 

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Discussion Starter #6
The # 1 o2 is the closest to the motor but dont be surprised if the code doesn’t go away right away. I replaced all of mine and it took about 6 months for the code to stop
-_- I really hope that isn't the case. Hopefully the obd scanner or disconnecting the battery helps. I just hope even with the cel in your case, it still works properly, so the car doesn't run with a faulty A/F ratio till it went away. Can you replace them all yourselves with normal tools? I'd rather do it all myself since the car will just sit at the shop for who knows how long. Thanks for the input George! I just wanna see that beauty run. A 4cyl Camry while reliable doesn't make me feel like I'm worth something to myself as did driving a 20+ yo Benz
 

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Premium Member
'97 CL600
Joined
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2,854 Posts
Thank you all for the responses. Can I get 1 type of cat and replace all 4? I'm just not sure if the primaries are a different shape or whatever. Like even for this, it says on Amazon it'll fit the front right but why wouldn't it fit front left and the secondaries? And if I do get something it'll be like this eh, with the slot for the o2s? Thank you for feedback. As a broke student, oem is out if the question! And I can run non-carb compliant in Texas. I'd assume those are more open flow cats so I'd prefer that. I read Walker makes great ones for the Benz too but I just don't want to buy cats and turns out they won't even fit.
View attachment 2612811
All four??? I think you have only two, one for each bank. Go to Walker or Magnaflow with your model number and year. They will tell you exactly what you need. You need professional advice not guesses.
Anziani
 

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Discussion Starter #8
All four??? I think you have only two, one for each bank. Go to Walker or Magnaflow with your model number and year. They will tell you exactly what you need. You need professional advice not guesses.
Anziani
Yeah just don't want to deal with people trying to sell me everything in the shop. But even when I looked on the magnaflow site it had 2 for each pipe for a total of 4. Which is what really sucks because the driver secondary was removed for being clogged but idk if that translates to the driver primary being damaged too! What a drag. A total of front and backs for both sides came up around $450 from Magnaflow. If I look at around 150-250 for installation in screwed.
 

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1999 S500; W140.051
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I believe the original '99 S500 sedan had 3 cats. 2 cylindrical cats; 1 ea mounted on the Left & Right banks on the 'downpipe' of the exhaust plumbing. There was also a gigantic cat under the car further downstream in the exhaust plumbing. The big one is the $1200 one from MB. It's the biggest cat I've ever seen. It sounds like the Magnaflow 'solution' is to replace the big one with 2 more cylindrical cats.

A look in EPC will show the original arrangement.
 
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