A fast shot and a low gauge reading are worth recording separately. Start by noting the coffee, grind setting and dose you were using when the change began. Don’t turn an internal pressure adjustment to compensate for a result you have not explained.
Keep the normal recipe in view before assuming a pump needs replacement.
Compare the behavior, not just one number
Record the displayed pressure during normal use, the shot result and which group you used. Does every group behave the same way? Does the reading change after idle time or during a run of drinks? Note an unusual pump sound without treating that sound as a diagnosis.
Do not use a generic target from the internet to adjust the machine. The model’s test procedure and operating specification determine what a reading means. A gauge alone does not prove that every part of the water path is working.
Check the operator side first
Review recent changes to coffee, grind and dose. Carry out the group-cleaning routine that your operator manual permits, then record whether the behavior changed. Do not remove an internal restrictor or valve to investigate it.
Also report a changed water supply, a recent cartridge replacement or a visible supply problem. Use the normal accessible checks in the model instructions. Don’t disconnect a hose or open a strainer as an improvised flow test.
What the technician needs to separate
The Linea PB flowmeter troubleshooting section includes coffee packed too tightly, blocked tubing, pump malfunction, a faulty valve and a damaged flowmeter among possible causes of lost flow. Its maintenance schedule calls for checking group flow, brew pressure and expansion-valve operation.
That supports checking the system before ordering a pump. On equipment with an OPV or pump bypass, the technician uses the correct design and specifications rather than assuming every valve performs the same job. Group components, supply conditions and internal restrictions remain part of the investigation.
The result should name the observed fault and the test behind it. A normal reading during one test does not establish that an intermittent problem has disappeared.
Leave the adjustments for service
Pump work, OPV adjustment, solenoid testing and boiler fittings belong to a qualified technician. If the machine leaks, overheats or has an electrical fault, keep it out of use and follow the manufacturer’s shutdown instructions.
Request diagnosis with the model, affected group, normal recipe and observations. If the problem began after service or cleaning, include that date and work as well.
Source: La Marzocco Linea PB manual, PDF pages 21–24 and 136. The flowmeter example gives possible branches for that model; it does not rank failure rates or prescribe pressure for other machines.