The module itself is a common failure, as are plugs (after a certain mileage/age). The failures are independent of each other, but you may only see plug failure codes even though it's actually the module that's failed, etc.
My car was showing only 1 working glow plug, so I replaced them all. After that all but 1 worked (apparently), but over a year another 2 'failed'. I replaced only the module and now all 6 plugs are fine.
Having read a few threads about it, maybe I'm wrong about the controller failing with no codes - seems it would normally throw at least one glow plug code.
I think shadow codes/inpa would be a good first step, as it's cheap and will be very useful in future.
P.S. Even through all my glow plug codes, which I had for years, and many short journeys (current commute is 2mi
), I've never had any DPF trouble (touch wood!!!), and the car runs very clean.