Boiler shows F85, both flow and return sensors implausible

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.

Possible Causes

Common cable damage, shared connector fault, incorrect sensor type fitted, PCB reference error

Safety: Allow boiler to cool before handling sensors.

Inspect harness: Check the shared wiring harness for cuts, burns, or rodent damage.
Connector cleaning: Remove and reseat the multi-pin connector on the PCB, ensuring no bent pins.
Sensor type: Confirm both sensors are genuine Viessmann parts with correct resistance curves.
Technician testing: Measure sensor resistances and compare to Viessmann tables; replace harness or PCB if necessary.

Hard 4/5

Sensor Wiring Harness

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