Crash course in TF O2 and cat operation

Hi All,
I’ve briefly mentioned this in a post by someone else relating to emissions.

TLDR:
How are the O2 sensors supposed to behave when being tested for MOT?

My 03 TF failed its MOT on emissions that apparently were waaaay out of tolerance. Around three weeks before the MOT I’d replaced the post cat sensor with a universal one from MGOC. This came with bare ends on the cables so it was joined to the original plug and cables with a short length (12 inch 1.5mm square) of copper flexible cables and crimped spades. This sensor had been the cause of the MIL light being lit for a good few months and my understanding of this is that it has no bearing on the operation of fuel/air mixture etc. So I wasn’t in a rush to replace it, but MIL light is a MOT fail so it was done. Once it had been changed the MIL light stayed off. I’ll also add that the TF was running perfectly - no rough idle, no lack of power, no stalling etc.
The MOT station said that due to the emission figures being so far out it must be the cat. So I duly ordered from ‘Bitz a cat, another O2 sensor and the gasket kit. The cat was fitted and it failed again on emissions. The post cat sensor was replaced by the Bosch one supplied by ‘Bitz, and the emissions were still out. They’ve now ordered and received another O2 sensor for the pre cat location and that is going to be fitted tomorrow. I’ve just been to enquire about the state of play and one thing they said that struck me was ‘on the machine ( presumably their MOT emissions gear) the pre cat sensor was up and down and the post was flat.’ My understanding of these things is that the pre sensor should be doing exactly this - up and down, the post sensor I’ve seen on my OBD reader real time mode doing both up and down and flat with the MIL lit before I replaced it.

Given that they’re going to change the pre sensor according to them that should sort it. If not what then? Change ECU? Bearing in mind the TF didn’t show any issues before the MOT.

Any thoughts on this would be appreciated, and apologies for War and Peace.
Regards,
Michael.

I seriously doubt the problem will be with the ECU, but sadly without having the car in front of me to do a diagnostic campaign I'm not going to be of much help.
I would however start checking through the system using an oscilloscope looking for inconsistencies. By using the scope, you could zero in on the problem signal to isolate and eliminate the problem.
Unfortunately most mechanics these days have absolutely no idea how to use one.
It's a bit complex to explain here, maybe I will get around to it, but I see little point as next to no one else here is likely to have an automotive oscilloscope.

Thanks Cobber,
As it happens I’ve got a ‘scope, not an auto one, just a Hantek DSO5012 for normal ‘tronics work. Is there any reason I can’t use that? If not, I’m assuming I’d need to get access to the two signal wires for the sensors.
Anyways, I’ll see what tomorrow brings…..👍🏻

You can use an normal oscilloscope , but it won't have the dedicated functions for automotive use, and it wont have the the special leads and adapters required to connect it.
it will take a lot longer to do a to work around.
You are looking for abnormal patterns from various components and the general wave patterns whist the engine is running. I've only ever used automotive oscilloscopes, over the years I've owned 3, still got 2 of them and some are a lot easier to use that others. They also come with proper instructions telling you what to look for with various tests that yuu can conduct.
The use of scopes pretty well died out with the introduction of OBD readers.
OBD readers are a powerful tool, that can diagnose problems very quickly, but are limited by the car designers ability to crystal ball all the likely problems that their design will encounter, so as codes can be assigned to each of those potential problems.
There are always with oddball problems that they didn't anticipate with every car design. This is where the scope comes into it's own.

An electronics head would be better able to guide you through the use of and adaption to the task of normal scope than I.
There are some here that I imagine would have a much better understanding of this than me.

I have an f & thus have one less sensor than the tf. The post cat’ sensor is there to ensure the catalytic converter is up to scratch & that the pre cat is feeding the appropriate information to the wizardry department & that department is adjusting the parameters around the car to ensure that when the information eventually comes back from the post cat sensor, a smile is formed if all is found to be good. The MOT tester gives you 12 months happy motoring, the sun comes out & the roof goes back.
That’s the theory & I guess that’s general knowledge.
Stepping closer to the system introduces values to aim for & these are referred to as “Stoick” values. This is the air/fuel ratio.
The ideal for a petrol engine is 14.7 to 1; one gram of fuel to fourteen point seven grams air.
The electronics strives to achieve this ratio but the initial information has to be close enough for the rest of the adjustable perimeters to be drawn in. The prime data comes from the Lambda sensor & the jiggery pokery that goes on in there gives a voltage ratio of 1.0 or the lambda value, or 1v max.
If your running lean expect that voltage to be as low as 0.1, a rich value would be not much less than 1 & so if you arrange a volt meter across the sensor where full scale is 1volt you will be able to see how rich the mixture is. Obviously if you’re on the rich side you’re going to trouble the tester more than on the lean side.
I did some research into this a while ago & ran a cable from the sensor to a narrow band meter temporarily mounted dash. In addition I arrange a breakout port beneath the trinket box under the arm rest, where I could bring out cables for better test gear, rather than rely on the device donated for these tests.
This space under the arm rest is the entrance to a parallel universe where various test points visit our dimension for me to play with; you can adjust your hand brake balance in there as well.
To sum up the pre cat sensor is the device which leads the pack, with everything else falling in behind with the ECU constantly referring back to that sensor.
For further reading on the Stoich value Google it & you’ll be amazed at all the stuff, however I found one error & that was repeated in most of the writings.
It wasn’t significant but it goes to show just how much plagiarism the internet creates in so called original bodies of work.
If you spot any errors in this please rectify, I’ve used most of this information to search out results & the answers corollate with expectations so I can’t be too far adrift.
M

Thanks Mike,
It’s all done, so as you’ve said “the sun comes out and the roof goes back.” A new cat and both sensors replaced is what was needed. Which begs the questions: did I fit a faulty post cat or were the additional lengths of cable I added enough to upset the resistance expected by the ECU? I remember seeing a YT video mentioning attempting to solder extra cable lengths to the sensor cables and it wasn’t possible as they aren’t tinned copper (can’t remember what metal they were), hence my use of spade connectors.
Next: new hood.
Cheers,
Michael.