Error FU

Safety precautions:

- If the boiler is displaying FU, do not keep attempting repeated resets. Repeated resets can stress components. A single reset is acceptable but if the fault returns, stop and call a Gas Safe engineer.

- Turn the boiler to the off position before you touch any external valves or the filling loop. If you are unsure how to isolate the boiler safely, switch the boiler electrical supply off at the fused spur or consumer unit and do not attempt internal repairs.

- Do not open the boiler casing, work on gas parts, or attempt to remove or replace internal components unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1. Note the display and fault: confirm the code is FU and when it appeared (during CH call, immediately on start, etc.).

2. Check isolation valves on the flow and return pipework at the boiler are open. Valves are usually lever-type (parallel to the pipe = open) or slotted heads. The error text specifically mentions checking isolation valves.

3. Check system water pressure on the pressure gauge. If pressure is low (below about 1.0 bar when cold), repressurise to the manufacturer recommended level (usually ~1.0–1.5 bar) using the filling loop following the boiler manual. After repressurising, use the boiler RESET once and observe.

4. Bleed radiators to remove airlocks. Start with the radiator nearest the boiler and work outwards, then re-check system pressure and top up if needed. After bleeding, restart the boiler once.

5. Ensure all radiator thermostatic valves and the central heating temperature knob on the boiler are open enough to call for heat.

6. Listen for the pump when the boiler tries to run (only with the boiler powered on and calling for heat). A working pump often emits a low hum; a seized or silent pump may indicate failure. Do not touch hot pipes if the boiler has been running—use caution.

Specific diagnostic and next steps if the above checks do not clear FU:

1. If the isolation valves were closed and you opened them, or if you repressurised or bled the system and the FU clears, monitor the system. If the fault returns, book an engineer—there is likely a deeper circulation or component issue.

2. If you suspect airlocks after bleeding but flow is still poor, try opening all radiator valves fully and operate the pump by calling for heat while watching for temperature change on flow/return pipes (careful, pipes can be hot). If the pump is running but circulation remains poor, there may be sludge or blockage in the system or the pump impeller may be stuck.

3. Do not attempt to remove or dismantle the pump, magnetic filter or heat-exchanger. Magnetic filters, sludge build-up and pump faults are common causes and require professional flushing, filter cleaning or pump replacement.

4. If the FU fault persists after basic checks (valves open, pressure correct, radiators bled, one reset) do not continue to reset. Arrange for a Gas Safe registered engineer to attend. The engineer will check pump operation, pump strainer/filter, flow and return thermistor locations and readings, and the PCB. They can safely perform power-isolated internal diagnostics, pump service or replacement, system flush, filter fit/cleaning, and replace thermistors or PCB if required.

When to call a professional:

- Call a Gas Safe registered engineer if the FU code does not clear after the basic checks above, if you hear no pump sound when the boiler is calling for heat, if you are not confident performing any of the listed checks, or if the boiler has repeatedly locked out. Internal component repair, system flushing and any work on gas connections must only be done by a qualified engineer.

Notes and cautions:

- The FU lockout protects the boiler from damage; treat it seriously and avoid prolonged use or multiple resets.

- Keep a record of what you tried and when the fault appears to give the engineer useful information.

- Do not attempt gas or electrical repairs yourself. If in doubt, stop and contact a Gas Safe engineer.