A4
Junkers
Flow Switch / Flow Sensor Fault (Heating Circuit)
A4 displayed, furnace does not start heating circuit
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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
Stuck flow switch, Blocked hydraulic block in Junkers furnace, Faulty flow sensor, Wiring issue to PCB
How to Fix / Troubleshooting
Safety first: Isolate power and allow the system to cool before opening hydraulic components.
- 1. Check pump operation: Ensure the circulation pump is running. A non-running pump can prevent the flow switch from activating.
- 2. Tap flow switch housing: Gently tap the flow switch or sensor housing on the hydraulic block to free a stuck paddle or magnet.
- 3. Clean strainers: If the furnace has strainers in the return line or hydraulic block, isolate and clean them to restore flow.
- 4. Sensor replacement: If the flow switch or sensor does not change state when flow is present, replace it with an original Junkers part.
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Repair Difficulty
Medium
3/5
Required Part
Heating Circuit Flow Switch / Flow Sensor
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