Subaru Legacy GT CEL code 66

There is a differential pressure sensor mounted beside the 2wd fuse link with a couple of 5mm pressure lines going in each end. These lines connect to each turbo output. The two pressures acting on each side of this sensor produces a voltage read by the ecm which tells it the differential pressure between both turbo boosts during acceleration. Normally the primary turbo leads the boost and the secondary turbo plays catch up until the pressures are even then air valves get opened and shut to bring the second turbo compression in parallel with the first turbo. The two turbos simply act in sequence but function in parallel. Its just like having one big turbo but operating down two smaller paths.
The sensor measures the changing boost pressures as the second turbo rises to equal the first.
If the generated voltage is not what it expects it shows as a code 66. As you can start to see there could be many different reasons why there might be an imbalance in the two pressures.
It could be either turbo causing the imbalance or a faulty bypass valve or pressure relief valve or a number of other issues. It doesnt necessarily have to be a turbo or a solenoid which is faulty. All it means is that the pressure difference between the two boosts is wrong.
The real question is why is it wrong.
There is a lot you can do yourself to diagnose the cause. If you are able to study the manual which is available free off the internet (do a search for the link) then you will see that you can test many of the component parts quite easily at home and also temporarily replumb the turbos to bypass aspects of the system control to figure it out for yourself. To do this you will have to gain an understanding of how the twin turbo system works. However its not hard if youi strip away the complex jargon hiding simple functions.. There is a mystique of fear concerning working on these motors which is not justified.Just use common sense and logic.
Or you take it to a shop and pay someone else to do it for you.