C2 P0420 but passed emissions test?

[Ikaron]

Hey there,
I have an issue with my C2 1.4i (petrol), which has somewhere in the range of 60k-80k miles. The check engine light comes on usually within 1 hour to 1 day after clearing with code P0420.

Issue is, it passed the MOT exhaust test with the following values:

Fast Idle @ 1950-2550 RPM:
CO: 0.22 % vol (limit 0.30)
HC: 6 ppm (limit 200)
Lambda: 1.017 (limit 0.970-1.030)

Natural Idle @ 600-950 RPM:
CO: 0.30 % vol (limit 0.50)

Additionally, we swapped the cats about 4 years ago, probably to some cheapo brand - I'd assume BM.

So, is it likely that the cats have already failed? It spent about 2-3 of those years just standing around not being driven, and about 1 year being driven multiple times a week, but probably < 50 miles a week on average. The driver also likes to keep the revs under 2k, could that have something to do with it?

The mechanic also said that he checked the entire exhaust for leaks, including at the exhaust manifold, and didn't find any.

I'm just perplexed, because surely these values mean that the pre-cat lambdas are working correctly and it isn't throwing a lambda-related fault code, and surely they mean that the cats are doing their job.

I also assume the post-cat lambdas work, as I said, no lambda-related fault code.

The only other thing I can think of, I saw someone mention somewhere that this can also be caused by a leak on the intake manifold, which I haven't checked yet. I guess that'd make sense as both lambda and HC seem to indicate that the car is running lean. Is that worth checking out?

Please let me know your thoughts, really not sure what to do with this.

[PaulC5]

This code may suggest a problem with the lambda sensor after the catalyst. You need to use a code reader with live data to see what values it is giving. If the cat is good its value should be steady, if moving similar to the inlet lambda sensor then it suggests a cat problem. You can also check its wiring. Without looking at live data you may end up replacing parts that are not needed and wasting money.