I just compared the figures on last years MOT to this years MOT (I know, far too much time on my hands!)

I'm wondering that last year, my HC idle ppm was just 21.0, but this year it shot up to 119.0

Is this a sign of some impending doom?

HC's are effectively unburnt fuel particles.

Can be a sign of oil getting into the engine.  But can also be a sign of fuel not burning completely in the cylinder, either due to poor atomisation (low fuel pressure/dirty injectors) or more commonly worn spark plugs giving too weak a spark.

When the cat is hot, it will burn some of those HC's, so it's possible that the engine was just much warmer on the previous test and fairly cool this time.

I wouldn't be concerned about it personally.  If you notice a decline in performance or an increase in fuel consumption before the next MOT then that would be the time to look further into it.  The MOT pass mark is 200ppm.

1 minute ago, TomsFocus said:

... more commonly worn spark plugs giving too weak a spark.

When the cat is hot, it will burn some of those HC's, so it's possible that the engine was just much warmer on the previous test and fairly cool this time.

Thanks Tom, all makes perfect sense. New spark plugs just months ago so doubt it's that. Last year we actually had a bit of a summer, so it was much warmer. This year I arrived very early so engine was just in idle for about 30 minutes, in addition to it being 15c outside, and pouring with rain!  Will bear in mind for next year to arrive with engine properly hot!

My recent MoT test experience shows that HC values can and do change, even when tested on the same equipment. Two different values back to back on the same afternoon gave 101 then 84, and the following day (cooler morning and after using a fuel treatment) 97.

Edit: Noticed that my values are for fast idle. Surprised that the readings arn't for the same test value.