Had the same problem myself. Changed the FSD unit but it didnt solve the problem. Read the manual - it reminded me that the control knob should be pushed in before turning. Have been doing this - doesnt solve the problem most times BUT if I turn the oven off after the (low) flame has been alight for a minute or so and immediately turn it on again all is well (may have to do this a few times). Wasted two FSD's before I discovered this. I was sidetracked into thinking that the FSD was faulty because it was possible to persuade the device to work by heating it with a blow-torch!
Ken Lewis
July 2008