The fact that the circulator makes noise or rotates does not mean there is sufficient water supply.
Actual flow
A clogged filter, closed valve, or air may restrict flow despite the pump running.
Flow switch
It may be stuck, incorrectly installed, or have an electrical issue.
Wiring
Check if the contact change is reaching the control board correctly.
Useful check
Comparison with an actual flow meter or hydraulic data helps to distinguish between a sensor fault and an actual hydraulic fault.
