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      06-29-2020, 01:11 PM   #1
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Mini Cat O2 Spacers Bad? Well Maybe...

When I installed my catless DP back in March I wasn't planning on tuning until I had the rest of the components installed, ie CP, intercooler, filter etc so I installed the mini cat O2 spacer to keep the check engine light off from having the catless DP. I ran this setup for about 3,000 miles without a check engine light and I would randomly check codes using my Bimmerlink app which never showed any O2 codes.

Fast forward to installing my other hardware and tuning I decided to just leave the stock cat box checked in MHD since the mini cat was doing it's job. I probably put another 2,000 miles on with multiple data logs etc watching LTFT and STFT along with all the other data, none of which pointed to any issues with fuel or O2 readings. As the weather warmed up, I started to notice the 12BB01 - Oxygen sensor after cat delayed response showing up when I would scan for codes, albeit still no check engine light.

A few emails back and forth with MHD about this as I had now unchecked the box to monitor the O2 and was still getting this code they sent some interesting information about the rear O2.

Essentially they had said to immediately remove the mini cat as there is possible damage that can occure to the engine. Apparently the rear O2 is not just a monitor for cat efficiency and does indeed have input into the STFT as a check of the wide band O2. It's a more accurate narrow band which acts as a secondary check and does indeed affect the fuel trims.

A good video I found while researching lots of post on the E90 forum in which they have discussed this topic to death.


So, I know I have advocated for these a little as a good way to pass emissions as well to keep the check engine light off for catless DP, but as it turns out they should not be run especially when tuned. Seems as the cars are pushed and driven hard, the minicat does not do it's job and the O2 then reads out of spec.

However, I do see that there is a benefit to them for short term emissions testing. When driven moderately they will keep the catalyst in the ready state and will not throw the CEL so if you are in a state where you cannot have 1 monitor not ready these are an easy solution to replacing the cat every year. I simply used a stubby 7/8" open end wrench to loosen and remove my O2 from the top of the car. Instead of completely swapping out cats every year one could simply install the spacer for a couple of days and remove when finished emission testing. In total I removed my spacer in about 10 minutes from the top alone.

Would love to hear others opinions, but for now I have removed my spacer with the MHD cat box unchecked and will put some miles on the car while monitoring LTFT and STFT as well.

Last edited by obeeone; 06-29-2020 at 01:50 PM..
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      06-29-2020, 01:25 PM   #2
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Can you share exactly what MHD said about the secondary O2 and how it affects fuel trims?
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      06-29-2020, 01:49 PM   #3
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Quote:
Originally Posted by thejeremyman9 View Post
Can you share exactly what MHD said about the secondary O2 and how it affects fuel trims?
Unfortunately they didn't go into detail about what exactly the rear O2 sensors do, but their exact quote was "Do not drive this MINI cat, the secondary sensors are needed for all kinds of measurement and you risk serious damage".

Then to just install the O2 back into its original position and uncheck the OEM Catted option box.

After reading numerous threads over on E90 post it's apparent that the rear O2 (narrow band) does monitor and provide feedback to the upstream (wideband) O2 which then adjust STFT and finally the LTFT.
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      06-29-2020, 01:58 PM   #4
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Quote:
Originally Posted by obeeone View Post
Unfortunately they didn't go into detail about what exactly the rear O2 sensors do, but their exact quote was "Do not drive this MINI cat, the secondary sensors are needed for all kinds of measurement and you risk serious damage".

Then to just install the O2 back into its original position and uncheck the OEM Catted option box.

After reading numerous threads over on E90 post it's apparent that the rear O2 (narrow band) does monitor and provide feedback to the upstream (wideband) O2 which then adjust STFT and finally the LTFT.
Ah OK. I am curious to know exactly how it is used, mostly because the DME probably expects it to have certain values based on having the OEM cat in place, and if you go catless or even aftermarket catted, the values it is seeing are going to be different, and as far as i know the tunes are not taking into account what type of cat you have or if catless, etc, aside from just the removal of the CEL. Kindof a long sentence i hope that made sense lol, but basically the DME would be seeing totally different values from the secondary O2 if you are catless vs. OEM cat, and i have not heard of the tunes accounting for that.

Also, narrrowbands are indeed quite accurate around 14.7:1 AFR but not really outside that range, so i question whether the secondary O2 is used at all during WOT situations.
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      07-03-2020, 08:24 PM   #5
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Quote:
Originally Posted by obeeone View Post
When I installed my catless DP back in March I wasn't planning on tuning until I had the rest of the components installed, ie CP, intercooler, filter etc so I installed the mini cat O2 spacer to keep the check engine light off from having the catless DP. I ran this setup for about 3,000 miles without a check engine light and I would randomly check codes using my Bimmerlink app which never showed any O2 codes.

Fast forward to installing my other hardware and tuning I decided to just leave the stock cat box checked in MHD since the mini cat was doing it's job. I probably put another 2,000 miles on with multiple data logs etc watching LTFT and STFT along with all the other data, none of which pointed to any issues with fuel or O2 readings. As the weather warmed up, I started to notice the 12BB01 - Oxygen sensor after cat delayed response showing up when I would scan for codes, albeit still no check engine light.

A few emails back and forth with MHD about this as I had now unchecked the box to monitor the O2 and was still getting this code they sent some interesting information about the rear O2.

Essentially they had said to immediately remove the mini cat as there is possible damage that can occure to the engine. Apparently the rear O2 is not just a monitor for cat efficiency and does indeed have input into the STFT as a check of the wide band O2. It's a more accurate narrow band which acts as a secondary check and does indeed affect the fuel trims.

A good video I found while researching lots of post on the E90 forum in which they have discussed this topic to death.


So, I know I have advocated for these a little as a good way to pass emissions as well to keep the check engine light off for catless DP, but as it turns out they should not be run especially when tuned. Seems as the cars are pushed and driven hard, the minicat does not do it's job and the O2 then reads out of spec.

However, I do see that there is a benefit to them for short term emissions testing. When driven moderately they will keep the catalyst in the ready state and will not throw the CEL so if you are in a state where you cannot have 1 monitor not ready these are an easy solution to replacing the cat every year. I simply used a stubby 7/8" open end wrench to loosen and remove my O2 from the top of the car. Instead of completely swapping out cats every year one could simply install the spacer for a couple of days and remove when finished emission testing. In total I removed my spacer in about 10 minutes from the top alone.

Would love to hear others opinions, but for now I have removed my spacer with the MHD cat box unchecked and will put some miles on the car while monitoring LTFT and STFT as well.
Were your emissions monitors ready?
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      07-08-2020, 10:11 PM   #6
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Quote:
Originally Posted by redbullfan View Post
Were your emissions monitors ready?
To be honest I don't remember checking all the monitors when I was running it, but didnt get any check engine lights for over 2000 miles driving without a tune and just the DP. It wasn't until I tuned the car and started really driving it that I got the slow response O2 code.

I am going to try to run it again right before my emissions this year in September and see how it affects the monitors to know for sure.
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      10-21-2021, 06:09 PM   #7
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Mini Cat Follow-up

I know it's been over a year since I posted about the mini cat, but my emissions did not require testing last year so I didn't get a chance to put the mini cat to test. This year however, testing was required so I decided to see how it works. Long story short, it worked perfectly. I took a couple of screen shots of the monitor ready states before the mini cat install, then the second is after 100 mile round trip to work with mostly freeway driving. We don't have visual inspection in Utah so the only thing needed is things in a ready state.







Essentially, this works as advertised for passing emissions. Just drive moderately after install as you don't want to be driving the car hard as the mini cat can affect fuel trims etc. Once you pass it's easy to remove, retune for aftermarket cat and drive worry free.

P.S. I removed and installed everything from the top using a stubby 22mm or 7/8 and took about 5 minutes for install and the same for removal.
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      10-24-2021, 08:45 AM   #8
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I'm not sure what the fuss is in these two spacer/mini-cat threads.

Is the sticking point really just "what does the secondary O2 sensor do?"

Mini-cats and spacers do work do get a car into ready state for emissions, but they only "work" by fooling the primary O2 sensor(s) - they do not actually change the composition of the exhaust gas mixtures present at the O2 sensor's location.

They fix one problem (emissions readiness) while creating another: false data.

The ECU will adjust parameters based on this information, so yes, MHD is correct that you wouldn't want a performance tune trying to adjust critical parameters using incorrect/false data as an input.

If you install mini-cats or spacers, return to the stock map and just drive the car normally (no roll-racing, drag strips, or stoplight-to-stoplight shootouts).

Do that and you'll be just fine.

Do not run mini-cats or spacers on anything other than the stock map.
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      05-27-2023, 04:06 PM   #9
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Just curious as to what brand of mini cat did you use and if this would work on an 2011 pwg n55?

Thanks
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