Hi Martin, merry christmas
I had a similar issue last month with my MOT, no physical issues with the car what so ever but it failed emissions, mine was around 6% when it needed to be 3.5%,
The guy doing the test let me mess with it and offered to test it straight away so I reset the idle and just wound in the carbon potentiometer, it then sat steady at 3.2% but when he ran the 15 second test it was about 3.6-3.8 then just dipped below in the last 3 seconds which was enough for a pass,
I came home and did some reading and found this thread on here which is very helpful:
https://www.polog40.co.uk/forum/index.php?topic=214.0I plan on messing with mine next spring and setting it up properly, I've got a new temp sensor as well and I'm gonna go round and check everything else but most of my engine is new anyway as it has been re-built so I suspect it might be down to the PPP chip i'm using - I'm tempted to try and track down a place that will do a proper rolling road tune but we'll see.
As for the blue temp sensor, on MOT day if I un-plugged it the revs dropped, which according to that thread means I might have faulty sensor somewhere? or does that mean the blue temp sensor is faulty? I had the car out of the garage the other day and it wasn't up to temp but I unplugged the blue temp sensor again and the revs stayed the same? I've got a new one anyway so I'll swap it out but doesn't make sense for that to be a cause of such high emissions.
Good luck with it, if you find any obvious cause can you post it up.