So if your theory is correct (i.e. any unaccounted for are stopped with faults) it would appear that they are picking up infinitely more faults doing periodic driver training than they are doing running up and down, day in, day out, in regular service.
It isn't a theory - I was trying to point out that it's possible some 195s aren't available for service for whatever reason and Northern aren't deliberately sending out a solo 156 in place of a 3 car 195. Anyway, Llama's post explains more
A fair few of those on Newton Heath are knackers for various reasons.
I don't think that
Llama's post really alters anything - it doesn't alter the fact that the vast majority of units currently not being used in service are those which have never been so used.
As I said above, to the best of my knowledge only thirteen 195/1s have seen public service, and, of those, ten are diagrammed for daily use - nine on a Sunday. Recently the number actually in service has averaged about nine - so that gives an availability of about 70%.
I've also said above that you'd be lucky to find a day on which there were more than six units engaged on training, etc. Well on Thursday of last week, for example, I could only find evidence of five 195s in use on training - of those two started at Huddersfield sidings, one at Wigan Wallgate Up Siding, one at Blackpool North CMD, and one at Doncaster. They may not have all been 195/1s, they may have included 195/0s, but even if we suppose they were all 195/1s, that means only five were in use out of the seventeen theoretically available - which gives an availability of about 30%. So, as I said previously, this implies that they are picking up infinitely more faults when engaged on training duties to what they are doing when in regular service - and that ignores the fact that, on a training day, a unit will usually only do about half the mileage it would do in normal service.
Unless, of course, Northern are deliberately keeping units aside..