VCDS O2 sensor aging and cat test results

[h4l]

I'm starting to diagnose the cause of an emissions MOT failure on my 1.6 mk4 Golf (AZD engine @ 158k miles). Failed on High CO during fast idle (measured CO was about 2x max).

I saw that a self-test of the O2 sensor and Cat can be performed with VCDS and gave it a try. Both the cat and O2 sensor test passed, but I'm not sure how to interpret the results. e.g. is it just scraping through or perfectly fine?

This is the first O2 sensor test I did, holding the revs @ approx 1700 rpm:


I noticed that running the test @ higher rpm resulted in a lower (worse?) result, but still "OK":


Still "OK" at 3000 rpm, but lower still:


The original post says:

The aging value must be above 0.80. The value for a new sensor is 1.99.

So it seems like the 0.64s value on the 3000 rpm test should have failed...


The Cat test passed with a value of 56.9%:

Is this 56% of the efficiency of a new Cat?

I've only had it for the last 70k miles or so, but I don't think the cats or O2 sensors have ever been replaced.

Thanks!

[h4l]

For what it's worth, these values are probably fine as I've resolved my emissions issue and it was not to do with the cats or lambda sensors.

The car was running rich when it failed. Long term idle fuel trim was -20% to -10% (no codes set though). While I was changing the air filter, I noticed the intake air preheater flap was fully closed to ambient air. I found and followed this test procedure for the thermostat controlling the flap, but the thermostat was not moving at any temperature (tried warming with a hair dryer but it stayed on 100% hot air from exhaust manifold, no ambient air).

This would have caused the rich condition as hot air would be less dense than cold air, so less oxygen. If the engine was adding fuel for denser air it'd effectively have too much fuel, making it run rich and back off on long term fuel trim. I didn't actually check the intake air temperature, but I guess it was too high for the intake temperature sensor to compensate for.

I wedged the thermostat so that the flap was 100% on ambient air. After clearing the learnt fuel trim, the long term idle trim stayed at -5%.

Took it to a garage to get just an emissions check done to see if it'd fixed it and got Pre-MOT test results below. Then took it back for a retest, which it passed, but not as well. The guy doing to retest didn't do much to warm up the cat, and had left the car off for 15 mins before starting the test, whereas the pre test guy gave it quite a few decent revs before testing, and the car didn't sit around before the test.

These are the results:

Description	Unit	Min	Max	Failed MOT test		Pre-MOT test	Passed MOT test
Fast Idle Test
Engine Speed	rpm	2500	3000	2640	2535	2570	2645
CO	%		0.20	0.66	0.75	0.02	0.17
HC	ppm		200	76	55	17	59
Lambda		0.970	1.030	0.992	0.979	1.001	1.004
Natural Idle Test
Engine Speed	rpm	450	1500	700		670	680
CO	%		0.30	0.49		0.00	0.45