Brew pressure is too low (below ~8 bar) and shots are under-extracted even with fine grind.

Safety first: Turn OFF and unplug the machine before adjusting the pump.

Steps:

• Check mains pressure: Ensure incoming water pressure meets spec. Install a booster pump if necessary.
• Measure brew pressure: Use a portafilter gauge to confirm low pressure.
• Adjust bypass valve: Increase pump pressure by turning the bypass adjustment screw according to manufacturer instructions until pressure reaches ~9 bar.
• Inspect expansion valve: If water is constantly returning to drain during brewing, the expansion valve may be leaking and should be cleaned or replaced.
• Replace pump if worn: If the pump cannot reach target pressure even with adjustment, replace the rotary pump assembly.