A few years out of the BMW game, so I was kind of outdated in the world of diagnostic tools. Now I got ISTA+ running and it is magical. Seeing that the front axle pressure sensor and the rear axle pressure sensors are calibrated the same. They also have the same voltage. However the front axle sensor is fluctuating from -12 to +110 bars with the engine off, no wonder theres a system failure. Seems new valve block would be the route to go at a dealer. However I found that the sensors seems to be available on eBay, Aliexpress etc for just $25-50. Seems they are commonly used as fuel rail pressure sensors on common rail diesel engines. They have the same part number, but there is a different number on the steel part of the sensor, so this is not confirmed information. I have already bought a new valve block, and I am still considering if I am going to replace it completely or try to order a new sensor.

Seems like a DIY job for anyone with proper diagnostic tools and a little bit of mechanical knowledge. Valve block have to be calibrated after replacing or changing anything on the system, but there's a procedure for that.

Main problem right now is that I dont have time to work on the car, working away in the weeks etc., so still considering letting the dealer handle this, but it would be a good feeling to conquer this one my self...