I just bought new used car(140k km) and turns out I have CAT gutted out by the person which sold me this car.. Car runs fine, no strange noises and doesn't feel numb or laggy for me. Only thing I noticed that it most likely running a bit on rich fuel mixture, I can smell not burned fuel sometimes, but only when I'm parking and jump out straight of the car after that.

I have driven 400km with it and got check engine light(yellow). I took a car to local service and they told me there is an error and I need to change NOx sensor. After changing it, they notified me that CAT converter is most likely freshly cut out.

I know that driving old cars without CAT doesn't change much this BMW N43 has all sort of "ecology" and other stuff, calculates fuel mixture by emission gasses parameters.

What is consequences for me driving like this? Is it better to find a replacement for CAT or is it not worth it? Or do I need reprogramming?

  • Welcome to Motor Vehicle Maintenance & Repair! Apr 5, 2022 at 17:57
  • If you have local/national vehicle inspections, your car won't pass the emissions tests without the cat. In some places, that makes it illegal to drive. In most places, it's also illegal to remove the cat. It's probably also illegal to sell a car with the cat removed. I hate to say "get a lawyer", but this might be a case for it - you're going to be on the hook for repairing it to get it past emissions testing. Catalytic converters are not cheap, which is why they are sometimes stolen off of cars in lots or at the side of the road.
    – FreeMan
    Apr 5, 2022 at 18:08
  • @FreeMan - Well, the reason they are stolen is for precious metal recovery. They get so much per cat by selling them to someone who reclaims the platinum in them. When stolen, they are not reused as cats. But, yes, cats can be expensive. We replaced them on my son's '03 330Ci (two cats on I6 engine). I don't remember it being overly expensive ... but then again, he paid for it. I just did the wrenching. Apr 5, 2022 at 18:16
  • Yes, I know they recover the platinum from them. My point, @Pᴀᴜʟsᴛᴇʀ2 was that they're expensive enough to be stolen, and that having to replace one on a vehicle that was sold without one (in order to pass emissions testing), might just be a reason to involve a lawyer. Plus, it might help shut down a scam sales place. If they were willing to sell one vehicle without the cat, they may be willing to remove them from every vehicle they sell, shafting all their clients. Maybe I'm being to skeptical...
    – FreeMan
    Apr 5, 2022 at 18:18
  • Thank you Pᴀᴜʟsᴛᴇʀ2. Yes I know about legal things in my country, but what I'm asking: is my car running rich fuel mixture, because I get bad mathematics now in my car PC without cat? I know i should replace it with used one, but i really don't want to spend extra money now, because NOx sensor was super expensive and also I had to change timing chain.. Apr 5, 2022 at 18:21

1 Answer 1


Depending on the engine, there will be (usually) two or four O2 (lambda) sensors. 1/2 of them are upstream while the other two are downstream of the catalytic converter. In most vehicles (some newer ones are different), the downstream O2 sensors are only there to monitor your cat's efficiency. Since you do not have any catalytic material in the body of the cat, I'm assuming this is why your O2 is throwing a code (what you see as a CEL on the dash). If all of this is accurate, there is no cause for alarm in driving your vehicle without a cat in it. It will not affect gas mileage negatively (may actually help improve gas mileage a little bit, lol). It won't damage your engine.

The biggest downside, besides the aforementioned emissions and legality issues (in comments), is if another code is to pop for some other reason, you won't know about it because the CEL is already illuminated.

EDIT: To further explain, it is completely up to you whether you need to find a replacement. If your country will allow the car to be operated without a cat, then have at it. It shouldn't hurt anything or affect how the vehicle runs. You can get the rear O2 sensors programmed out, or there is an option to get fake O2 sensors which will continually feed the ECU information it needs to believe the cats are functioning normally, which will turn off the CEL.

  • Thank you for answer. But I think my question is more about this: bimmerprofs.com/n43n53-operation-modes-basics how cat removal effects operation modes of this engine. I think I will have to find someone who cat wire up diagnostics tool and monitor parameters while driving Apr 6, 2022 at 11:53
  • It appears you didn't fully read what I put into my answer. See the highlight in my answer. Apr 6, 2022 at 14:47

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