The most common issue in the Sage Pro and Sage Touch is that the boiler gets blocked, sometimes limescale blocks it and other times it just gets blocked. As the holes in the boiler are so small it is impossible to unblock them so you have to replace the whole boiler. The process is pretty easy. Get the top off the machine, then only remove the metal disk (boiler) don't remove the plastic boiler holder as there is no need and it will make the job harder.
Another issue with the Barista PRO is error "er" this youtube video shows it. This kit contains the part you need to fix that error.
Boiler will fit the following models:
Part No. SP0028450