Reading from other sources and now from personal experience, it turns out that if the front camera is somehow malfunctioning it will also take down the rear one, shutter and vice versa. As a solution I'd simply suggest purchasing a few front and rear cameras and make them part of your replacement process when you do have these failures. The two cameras can be bought for under $3 each. though it sounds like a lot to a regular business, at least it stops you building up a lousy reputation.
Later.