Bad exhaust gasses / emission test failed


Oct 15, 2020
After removing the mufflers and pressure-tested the cats/headers I felt air coming out of the collector of the passenger's header.
I had a set of new aftermarket headers at home so I installed them. It was a pain to get the stock h-pipe sealed to these headers on the passenger's side but with the help of high-temperature silicone, I could manage it. I pressure-tested the entire exhaust system and also used soap water on the joints. I had tiny bubbles showing up at the ball joints between the h-pipes and the mufflers. I did not seal them as I thought this might not affect the emission test. All other joints did not show any bubbles.

In the meanwhile, I also set the timing to 14 degrees BTDC.

Yesterday I was back at the MOT to get the emission test done - again.
Side info: The tank was filled 50% with 95 octane (that is the lowest we can get here).

Test 1: failed.

I changed the timing back to about 8 degrees (before I changed to 14 it was somewhere in that 8 degrees area).
The values got better but the CO2 level is still a tiny bit too low and the O2 value way too high:

Test 2: failed.

Values @ idle:
CO213.07 %
O22.2 %
HC26 ppm
COno values as they do not test CO

I pulled the codes after the test: nothing -> only 11

So the values are not too far away from where they may be but these values tell me I am still too lean and/or have still false air coming inside the exhaust system.

Would it reduce the O2 in the exhaust gases if I would remove the smog pump hose? Or will this make the entire emissions even worse?

O2 sensors may be faulty?

Any other ideas?

I did check the temperature after the first cats with a temperature gun: the passenger's side was slightly hotter (about 65°F).

I am quite desperate as this is the last step to get her back on the road...
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