check the solinoid it should be 3.5 ohms or above if below it's faulty.
also check the shower remove the shower head and turn to cold does it come out fast, then turn to hot does the flow slow right down ?
if they are both the same hot & cold the flow valve /stabilizer is faulty.
the filter is the plastic sleeve you can see up the water inlety.
hi-spec plumbing.
September 2007