F3
Saunier Duval Communication Failure with Boiler (eBus/OTBus)

Thermostat shows F3 and cannot control the boiler correctly; heating does not start or stops unexpectedly

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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

Open circuit in eBus/OTBus wiring between thermostat and boiler, Reversed polarity or incorrect connection on boiler bus terminals, Faulty boiler communication board, Faulty thermostat communication interface

How to Fix / Troubleshooting

Safety: Isolate mains power to the boiler before touching any wiring. The bus lines are low voltage but are inside the boiler casing.

  • Check boiler status: Confirm the boiler itself powers on and shows no lockout code. If the boiler is off, the thermostat cannot communicate.
  • Inspect bus wiring: At the boiler, locate the Saunier Duval eBus/OTBus terminals (often marked eBUS, BUS+, BUS−, OT). Ensure the two-core cable from the thermostat is firmly connected and not mixed with 230 V terminals.
  • Continuity test: If possible, disconnect both ends and test continuity of each conductor with a multimeter. Repair or replace damaged cable.
  • Polarity: Some Saunier Duval smart thermostats are polarity-sensitive. Swap the two bus wires at the thermostat end and test again (only if the manual allows this).
  • Reset pairing: On the thermostat, run the boiler pairing or communication setup wizard again.

If F3 remains and wiring is correct, the boiler's communication board or the thermostat interface may be defective; contact a qualified technician.

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Repair Difficulty

Hard 4/5

Required Part

Saunier Duval eBus/OTBus Communication Cable
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