The flow sensor on Westinghouse tankless (WGRGH gas and WET electric) detects water flow and signals the control to activate heating. An impeller spins as water flows; a hall-effect sensor reads RPM.
Symptoms of failure
- E01 (gas tankless) or E1 (electric tankless) error despite verified water flow
- Intermittent activation — heats sometimes, doesn\'t other times at same flow rate
- Fails to activate at low flows that previously worked
- Delayed activation — heating starts several seconds after flow begins
Diagnostic before replacement
- Verify actual flow at the unit inlet — bucket test confirms flow is present
- Clean inlet filter — clogged filter mimics flow sensor failure
- Inspect impeller for debris or stuck condition
- Test sensor with multimeter on the hall signal lead during flow
Common cause: stuck impeller
Debris from new construction or hard water lodges in the impeller chamber. Cleaning resolves without replacement:
- Power off; close inlet shutoff
- Open hot tap to relieve pressure
- Disconnect flow sensor housing
- Remove impeller; clean with soft brush
- Reinstall; restore water and power; test
Replacement
- Power off; close inlet shutoff; relieve pressure
- Disconnect sensor leads at control board
- Remove sensor housing
- Install new sensor with new gasket/o-ring
- Reconnect leads
- Restore water; verify no leaks
- Restore power; test flow activation
Cost
- Flow sensor (OEM): $60-120
- Contractor labor: $150-250
- Within 5-year parts warranty
Bottom line
Most "flow sensor failures" are actually clogged impellers. Clean first, replace second. Westinghouse tech support can guide diagnosis over phone.