Error F.10

Safety precautions:

1) Before touching the boiler, switch the boiler off at the programmer/room stat and isolate the electrical supply at the fuse/consumer unit. If you know how, isolate the gas supply to the boiler. Allow the boiler to cool completely. Do not work on live electrical circuits or gas parts unless you are a qualified engineer.

2) Wear gloves and eye protection and keep the area dry. If you smell gas or suspect a gas leak, leave the property and contact the gas emergency service immediately.

Initial homeowner checks you can do (no tools or minimal tools):

1) Try a simple reset: follow the boiler’s reset procedure (consult the user manual) and see if the error reappears. If it returns immediately or within a short time, proceed to further checks.

2) Visually inspect the flow NTC location: the flow sensor is clipped to the flow pipe inside the boiler. Look for a loose or unplugged connector, a sensor that has fallen off its clip, obvious water corrosion at the plug, crushed or chafed wiring, or signs of burning at the connector/pins.

3) Check the cable routing: follow the cable from the sensor as far as you can see. If you find pinched, cut, or crushed insulation, that is a likely cause. If everything looks intact and the connector is plugged in, record that information for the engineer.

If you are comfortable using basic electrical tools and understand isolation procedures, further diagnostic checks (ONLY if mains power and gas are isolated):

1) Accessing the sensor connector: remove the boiler outer casing following the manufacturer’s instructions (if you are not sure how or uncomfortable doing this, stop and call an engineer). Locate the flow NTC connector on the wiring loom and the PCB.

2) Disconnect the flow NTC from the wiring harness so you measure the sensor directly, not through the wiring or PCB. Use a digital multimeter set to resistance (ohms).

3) Measure the resistance across the two pins of the flow NTC at room temperature (around 18–25°C). Typical NTCs used by Vaillant are approximately in the 10–12 kΩ range at ~20°C (values vary by model). A very low reading approaching 0Ω indicates a short circuit. An infinite/open reading indicates a disconnected or broken sensor (different error codes normally). A reasonable value in the expected range indicates the sensor is likely OK and the short is in the wiring or PCB.

4) If the sensor resistance is near zero (short), remove and inspect the sensor and plug for water ingress or corrosion. If you are competent to replace just the sensor, you may replace it with the correct Vaillant part, ensuring you fit and secure the sensor properly to the flow pipe and re-route or clip the cable to avoid chafing. If you replace the sensor, reconnect and test by restoring power and checking whether F.10 clears.

5) If the sensor resistance is normal but the boiler still reports F.10, the wiring harness may have a short between the sensor connector and the PCB or a short to earth. With power isolated you can continuity-test the wiring harness for shorts to earth/ground and for continuity between the sensor plug and the PCB connector pins. If wiring is intact, the PCB connector or PCB may be faulty.

When to call a professional:

1) If you are not confident performing electrical isolation and resistance checks, call a qualified Gas Safe engineer. Any work involving gas connections, pump removal, or replacement of components inside the combustion chamber or PCB must be done by a registered engineer.

2) Call a professional if the sensor reads shorted, wiring is damaged, or the fault persists after replacing the sensor. A Gas Safe engineer will have the correct replacement parts, will confirm the correct resistance values for your specific model, test and replace wiring or PCB components as required, and run the necessary safety and performance checks after repair.

Other notes:

1) Do not repeatedly reset and run the boiler when an NTC short is present — this prevents proper safety operation and may mask a failing component. If the boiler is not heating and the fault persists, avoid using the boiler until it is checked.

2) Keep a record of the fault code, any tests you performed and the readings you obtained — this will help the engineer diagnose and reduce call-out time.