Air con problems

[Proman]

i been having a problem with the air con for a while now, when i turn the selector knob it just makes a hissing noise, its like it dont change, like its stuck just blowing air through the front vents, it still get pretty cold but yeah if i put it to a different setting it just hisses and doesnt go away???

[hako]

If you've been playing under the dash you may have knocked a vacuum hose off - have a look under there with a torch and engine running. The actual vacuum switching unit behind the knob also has a habit of breaking and when this happens it can do just as you describe.