Error F.1

Safety precautions:

- Do not remove seals or internal covers unless you are a qualified Gas Safe engineer. Live electrical parts and gas components are dangerous.

- Before touching any wiring or components, isolate the boiler from mains power at the fused spur and, if instructed by an engineer, turn off the gas supply. If in doubt, leave isolation to the engineer.

- If you smell gas, evacuate the property and call the gas emergency number immediately.

Initial checks a homeowner can safely perform:

1. Record the exact fault code and any other codes displayed. Some boilers cycle through multiple codes.

2. Try a single reset following the manufacturer instructions (consult the user manual). If the code returns immediately or after a short run, do not repeatedly reset; record behaviour and stop.

3. Check the heating system pressure on the boiler display: target ~1.0–1.5 bar when cold. Low system pressure can cause faults and should be topped up per the manual. If topping up was recently done or the system was drained, note that connectors may have been disturbed.

4. Visually inspect external wiring and visible plugs: look for an obvious disconnected plug, a loose connector or water/condensate on external wiring. Do not open the boiler casing to inspect internal wiring unless you are qualified.

If you are mechanically and electrically competent and choose to proceed (only when the boiler is isolated from mains power):

5. With power isolated, check the plug connection for the return temperature sensor where it connects to the wiring harness (usually at the return pipe/heat exchanger or a labelled connector). Reseat the connector if visibly loose or corroded.

6. If you have a multimeter and know how to use it safely, measure the sensor resistance at the sensor plug. Vaillant NTC sensors typically sit around about 10–12 kΩ at 20°C (values vary by model). An open circuit (infinite/OL) indicates an interruption; a very low value indicates a short. If the reading is out of range, the sensor or wiring is faulty.

7. If the sensor reads correctly at the plug but the boiler still reports F.1, the wiring between the sensor and the PCB or the PCB input may be faulty. Continuity testing of the harness from sensor plug to PCB and voltage checks on the PCB inputs should only be carried out by a qualified engineer.

Recommended professional diagnostics and fixes (to be performed by a Gas Safe engineer):

- Confirm the fault with diagnostic tools and check fault history in the boiler menu.

- Test and measure the return NTC resistance at ambient temperature and compare to specification. Replace the NTC if defective.

- Inspect and test the wiring harness and connectors for breaks, corrosion or water ingress; repair or replace damaged cabling and connectors.

- Check the PCB sensor input for damage or loose terminals; replace PCB if proven faulty.

- After repair, run the boiler through a full call for heat and monitor temperature readings, flow/return differential and pump operation to ensure the system is stable.

Final note: If F.1 persists after basic visual checks and a safe reset, call a qualified Gas Safe engineer. Do not attempt internal electrical or gas repairs yourself. A professional will have the correct tools and certification to diagnose and repair sensor, wiring or PCB faults safely.