Post Cat O2 sensor saga…. Continued

 Hi All,
As per my previous posts on this:
TLDR
Jan(ish) 24
MIL light on; codes say post cat sensor U/S.

April 24 replace post cat sensor from MGOC. MIL disappears.
May 24, MOT time arrives; fails badly on emissions. New cat (one sensor and cat from ‘Bitz) and BOTH sensors (second sensor ordered by tester from unknown company) replaced by tester to get through.

Now got MIL back with code P0420 - Cat not performing too well (or words to that effect).
see the attached live data from last journey with MIL lit.
Your thoughts on the way to resolve this issue would be greatly appreciated.
Cheers,
Michael.




 

A couple of aspects occur. Even though there shouldn’t be a question as to the parentage of the lambda sensor, when one considers the device was supplied by the MGOC, it could possibly have been suspect. It is however problematic with the MOT tester fit unit. The sensors are a Bosch designed item & as such all the parameters around which the electronics is optimised will be within those values laid down in the manufacturers specifications. 
I did some work a while ago on a design to monitor the stoich point (optimum point) for maximum efficiency. I’ll include a picture of a device I made to connect to a breakout point beneath the arm test, this is a very sensitive volt meter & NO I won’t make you one! It isn’t just a black box with a volt meter in it, it a black box with an instrument amplifier & a volt meter. 
The values from different sensors was interesting but only the Bosch sensors were consistent. I would rather use a second hand Bosch than a new pattern part. 
Number two, we don’t drive our cars hard enough & for long enough to get even the best catalytic converter hot enough to burn away the grot it accumulated whilst we potter around  with cold engines & whilst stuck in traffic. 
I’ve noted on here for the last few years that the emissions from my car haven’t troubled the tester, that’s because before offering it up for test I do a reasonable number of hot miles. 
If these thoughts do not chime with all of you, I must point out that my car hadn’t had an emissions fail in a goodly number of years. 
m   

Thanks Mike,
Your second point definitely applies to my MG! Almost to the point of asking myself ’why have I got it?’ Although I asked the same about my Jag XK8. Another thread perhaps.
The fact that there’s never been anything resembling adverse performance since this all kicked off pulls me again towards your second point. Thing is, after my Jag experience - scratched from passing motor and tyres spiked because I ‘parked where I shouldn’t have’ according to someone has given me a complex about using it and leaving it in car parks. This is not helped by an experience in my Freelander of having the OSR door smacked while I was sat in it! Apparently I’d parked ‘too close’ even though I was further away from the parking space marker than she was. Yet another thread subject.
Anyway, will go back to the tester and find out where they got their sensor from and maybe replace it, but until the motor starts playing up I might just leave it. 🤷‍♂️
Regards,
Michael.

If it helps, this page from a website I use looks good. It confirmed that the downstream O2 sensor wasn't working as it should, I was getting the same readings from both, which can trigger the MIL.
https://www.obd-codes.com/p0420

Thanks Pharg for your reply.
Interesting that the site you linked to in its explanation of O2 sensor operation stated that the post cat sensor should be some reasonably stable value. Other sources state that it should oscillate as per the pre cat. My confusion continues having seen both types of traces from the post cat - the one above recently and the oscillation when it was freshly replaced.
Anyway I’ve just had a 40 odd mile blast and there was no sign of the MIL. I’ve also made a note of the mileage it came on at the other day, so I’ll see if what the mileage difference is if it returns. Some sources have mentioned around 100 miles and/or a few start stop cycles between the MIL showing itself. All in all it working perfectly so I can’t see what the problem is (famous last words….).
Cheers,
Michael.

Maxthedog, I bought my TF from a well-known dealer. WHen I drive it home, the MIL came on.
The dealer sent me a replacement ECU which had the MIL light switched off. (Symptom solved).
The dealer had installed a new CAT.

Recently, I had the Kmaps remap done by Mark (a brilliant improvement to driving and everything about the car).
Mark was the fuel and emissions engineer at MG-Rover when these cars were being built. He wrote the book.
(And a nice bloke too).

Mark explained that aftermarket CATS will cause the MIL to light and it is related to the design of the CAT.
The CAT will pass an MOT (although make sure you take it for test when fully warm or hot).but will not extinguish the MIL.
It may be possible to buy an NOS CAT from somewhere (maybe Rimmers) but at many hundred of £s compated to less than £200 for the aftermarket.

It looks like switching off the MIL in the ECU is the only option with an aftermarket CAT.
I think it is related to the distance between in and out O2 sensors on aftermarket CATs but I cannot confirm it.

I do recommend calling Mark to answer all your questions.
I believe the above to be correct but welcome learning more if it is not. And good luck.

Good luck with it!
I've been trying to fix an intermittent MIL for a while now, and after replacing the post- Cat Lambda, I'm now getting a different reading , i.e. a different voltage from the post -Cat compared with the pre-Cat, which I wasn't seeing before, so I'll just wait and see.

Hi,
Generous Dad, thanks for that info. The engineer you mentioned, could you PM me his contact details if you don’t want to reveal which well known dealer it is.

Pharg95
Given that nothing untoward is happening - stable tickover, no power dips and anything else that might point to a more pressure issue, I’ll just see how it goes too. I was speaking to a car restorer whos specialty is Austin Healey about it today and he agreed. The TF certainly doesn’t lack any performance!
Cheers,
Michael.

 This sounds amazing as I found it very much useful
and informative to be honest. Also, I have gone through this post which
definitely helped me out a lot as a new member I am looking forward for more
such discussions.