Failed MOT, high CO

[Robert986]

Hello boys! (and girls?)

I could use some pointers.. Today I failed the MOT om high CO :ah: box 2003 986s the story goes as follows:

2019 oktober: Car basically oem, everything running fine. Standard headers, cats (only rear cats here in Sweden) standard catback.

MOT-values
HC: 5 (200)
CO: 0 (at idle, limit 0,3)
CO at 2500 rpm: 0,0 (0,2)
Lambda: 1,01 (0,97 - 1,03)




Winter/spring 2020: switched to long tail headers (ebay), 200-cel cats (ebay), Brombacher sound version catback. After this I got a cel and the following codes:
p0441 (tank vent system)
p0159 O2 sensor ageing behind cat bank-2
p0158 O2 sensor behind cat bank-2
p0430 Cat conv efficiency bank-2
There was also a code related to o2-sensor ahead of cat bank-1 (don´t remeber which)

Initially there was some minor leaks that I fixed, I replaced the o2-sensors ahead of cat bank-1 and after cat bank-2.

After this I drove the car för some weeks, I noticed that p0430 kept coming back, I also notice a quite noticeable sooting om the right exhaust pipe (bank-1).

Yesterday I cleared code p0430 again, this time I also hade the P0139 (aging o2 sensor after cat bank-1 for the first time)

This morning I started the car, noticed that it smoked maybe a little more than normal but it was rather cold outside. All codes cleared, went to the MOT:

MOT-values
HC: 90 (200)
CO: 0,1 (at idle, limit 0,3)
CO at 2500 rpm: 0,7 (0,2) (This was measured on bank-1, bank-2 gave CO 0,5)
Lambda: 1,01 (0,97 - 1,03)

So I failed..

All in all, something is not right. I could suspect the new cheep ebay 200-cel cats, but why so much soot on just bank-1 exhaus pipe.. And what I understand the ebay-200-cel cats normally do work.

The plan was to put on my new intake-stuff IPD 74TB, 987 airbox and all, then I have a appointment at the tuner in oktober. But this cannot happen if the car doesn´t go as it should before the tune.

It feels ok to drive, no hesitation etc.

My plan:
-Replace the o2-sensors that arent replaced (bank-1 after cat + bank-2 before cat)
-Check sparkplugs, can a sparkplug have come loose and cause a air leak on one bank?
-Open up the intake, plenum etc and see if there is oil residues there. If oily, switch AOS.
-Clean MAF (Or should I just buy a new one? it´s a bit hard to test since the car feels like it runs fine)

I would appreciate all kind of ideas on how to track this problem down, I have Durametric so is it anything I should test?

A long-shot: I take it that if the ECU doesn´t see that the car goes up to operating temp, then it remains in open loop. Can this be verified with Durametric? On the temp-meter in the cluster it looks like it is up in temp.

Sorry for the looong description :-)