We have a self-cleaning range which wont self-clean.

Some time ago the interior light refused to switch off, but we decided
to just let it burn itself out and thought no more about it. However
the range now refuses to lock the door when we try to use the
self-clean function and I'm wondering if this could be the reason.

The light switch looks ok but I tried disconnecting the power to it
just in case, but it made no difference.

The fuse is ok.

Has anyone seen this occur before and could it be the reason the door
wont lock, or is it more likely the servo motor which needs replacing?

Any other ideas??

Steve