F.11 displayed; unit may start briefly then shut down, or not start at all

Safety: Isolate electrical and gas supplies. Allow the appliance to cool.

• Inspect the NTC location: Check for leaks around the return sensor pocket or pipe. Any sign of water can cause shorts; repair leaks before replacing components.
• Check the sensor: Disconnect the return NTC and measure resistance. A near-zero reading indicates a short. Replace the sensor if shorted or out of spec.
• Examine wiring: Look for melted insulation or wires touching each other or the chassis. Separate and insulate wires, or replace the harness if damaged.
• Dry connectors: If moisture is present, dry thoroughly and use contact cleaner. Ensure a snug fit when reconnecting.
• Reset and test: Restore power and gas, reset the appliance, and run a hot water draw to confirm the fault is cleared.
• Ongoing F.11: If a new sensor and sound wiring do not resolve the issue, the PCB may be faulty. Have a Vaillant-trained technician test and replace the board if required.