What's your experience with MoT emissions failure?

This seems to be something that a lot of folk have problems with, so I thought maybe it would be helpful if there was a thread where we could find what causes test failures and how to fix things to get a pass. (Not by offering a bung, by the way!)

Over the last few years I've had over-high readings and have fixed it by getting a new cat, but this year it had no effect. I'm interested therefore in finding out what else I have to do this time.

So far I've done this -

another new cat - no change.

set of new plugs - no change.

new O2 sensor - no change.

Next actions are to improve the exhaust joint seals, as there are only very slight blows with a bit of condensate coming out as it warms up.

Starting to tear what little hair's left, so what else should I be trying?

Can the water temp sensor be faulty without showing a warning?

Are my 10-year-old HT leads past it?

So how have you sorted the problem???

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