303
Ariston
Low Water Flow / Flow Switch Fault
Error 303, furnace indicates low water flow or no circulation.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked system, Stuck flow switch, Air in system, Failed circulation pump
How to Fix / Troubleshooting
Safety: Allow the system to cool before bleeding radiators or opening valves.
- Check system pressure: Confirm the pressure gauge is in the normal range. If low, top up via the filling loop and bleed air from radiators and high points.
- Bleed air from pump: Some Ariston pumps have a bleed screw. With power OFF, open the screw slightly to release trapped air until water appears, then retighten.
- Open all isolation valves: Ensure the valves on either side of the furnace and pump are fully open.
- Inspect flow switch: Locate the flow switch or flow sensor on the return or flow line. Check for external leaks or damage. Tapping gently on the body can sometimes free a stuck paddle, but if the fault recurs, replacement is needed.
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Repair Difficulty
Medium
3/5
Required Part
Flow Switch / Flow Sensor
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