Error F.11

Safety precautions:

1) Do not remove boiler casings or attempt internal wiring or component repairs unless you are a qualified Gas Safe engineer. Internal work exposes live electrical circuits and gas components. Working on gas appliances without registration is illegal and dangerous.

2) Before doing any visible checks, switch the boiler off at the programmer and the mains isolation (or the isolator switch) and, if possible, isolate the gas supply. Only carry out non-invasive visual checks with the unit powered down.

3) If you smell gas or see obvious water leaks, evacuate the area and call the emergency services or your gas supplier/engineer immediately.

Initial checks a homeowner can safely do:

1) Note the exact error and any recent work: record F.11 and whether it happened after a service, drainage or pipework work. A disturbed plug after a drain-down is a common simple cause.

2) Try a simple reset: switch the boiler off at the programmer then at the mains for 1–2 minutes, restore power and see if the fault returns. If the code comes back immediately, do not keep resetting.

3) Visually inspect accessible external wiring and the pipework around the boiler: look for damaged, crushed or wet cables, loose connectors, or signs of corrosion at the external sensor plugs (do not remove panels). Check the sensor leads where they run on the outside of the boiler or on the pipes for chafing or kinks.

4) Check for water ingress: if condensation or water is visible in connectors or on wiring, that can cause a short – note this for the engineer.

Diagnostic and fix steps a qualified engineer will perform (do not attempt unless qualified):

1) Verify fault memory and run any manufacturer diagnostic menus to confirm F.11 and any related faults (e.g. F.01 open-circuit variants, F.64 electronics).

2) Isolate mains and gas, then remove the boiler cover and visually inspect the return NTC connection, wiring harness, and PCB connector. Look for burn marks, loose pins, corrosion or moisture.

3) With the sensor unplugged from the PCB, measure the NTC resistance at ambient temperature using a multimeter and compare to the manufacturer's specification (Vaillant return NTCs are commonly around 10 kΩ at 25°C but confirm the exact value for the model). A near-zero reading indicates the sensor is shorted; infinite/very high indicates an open circuit (F.01).

4) Check continuity and insulation of the wiring harness between the sensor and PCB. Measure for short to earth or short between conductors. Wiggle the cable and connectors while measuring to find intermittent shorts.

5) If the sensor and wiring are good, test the PCB input for short or board faults (measure at PCB connector for unexpected voltages or shorts). Replace PCB only after confirming harness and sensors are OK.

6) If a defective sensor is found, replace it with the correct Vaillant-approved NTC sensor and ensure it is clipped to the correct pipe (return pipe) and routed without tension or contact that could chafe the cable.

7) If the wiring harness is damaged, replace or re-route and secure it; repair plugs or replace connector housings as required. Ensure all connectors are dry and fully seated.

8) After repairs, clear fault memory, power up, and run the boiler through a full fire and monitor the return sensor reading and behaviour. Check for stable readings and that no further short is detected.

When to call a professional:

1) If the fault returns after a reset, if you find damaged wires or wet connectors, or if you are not comfortable with visual checks, call a Gas Safe registered engineer. They have the training and diagnostic tools to safely trace shorts and work on gas/electrical components.

2) Any internal measurements, sensor replacements, PCB work or gas-side work must be done by a qualified engineer. If you have any doubt, book a visit. The engineer will complete safe diagnostics, replace genuine parts where required, and test the system thoroughly before leaving.