The F9 fault code indicates an issue with the water pressure sensor, filter or pump malfunction.
What’s bad about this code is that it can indicate different issues, which include unstable circulation, pressure, and a diverter valve problem. To locate the problem, you should check the pump and the entire circuit board. The sequence may be incorrect.
In my own personal experience, and usually also with communication from Glow-worm, this fault code appears when the plate heat exchanger gets sludged up. To prevent this, the system should be cleaned and flushed regularly.