Furnace displays F9, no heating, may occur after power surge or wiring work

Safety first: Turn off power before working on control wiring. Low-voltage circuits can still be damaged by shorts.

• Check external controls: If using a Saunier Duval eBUS room thermostat or controller, verify that it powers up and displays correctly. Replace batteries if applicable.
• Inspect wiring: Open the control terminal block on the furnace and check that thermostat/eBUS wires are correctly connected as per the wiring diagram. Look for loose strands or shorts between terminals.
• Test without external control: If possible, temporarily disconnect the external controller and fit a simple link (if allowed by the manual) to simulate a call for heat. If F9 clears, the issue is likely with the external device or its wiring.
• Bus polarity and type: Ensure that only compatible Saunier Duval eBUS devices are connected to the eBUS terminals and that polarity is correct where required.
• PCB replacement: If wiring and external devices are confirmed good, the main PCB’s communication circuit may be defective and require replacement by an authorized technician.