Error code 42

[tenbabs]

Hi my engine warning light came on. I ran the diagnostic and got error code 42

The car occaisionally stalls when, I go down a gentle slope, go down the gears, go into neutal and am just applying the brake. - only when going down slope

I had a mechanic look at it who suggested idle valve problem. He suggested temporarily taping off one of the small black frexible hoses which was not connected to anything (? idle valve)

Would this be the no. 1 speed sensor ?

Thanks in advance
P.