Glow-worm F56 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F56 fault code mean?
The F56 code appears when your Glow-Worm boiler detects a problem with how it monitors or moves water around your heating system. Specifically, it points to one or more of the following: a faulty or disconnected flow or return temperature sensor (NTC thermistor), a wiring fault on those sensors, a circulation problem caused by the pump, a stuck valve, or a blockage, and in some cases a defective internal expansion vessel. Because the boiler can no longer confirm that water is flowing at the correct temperature, it locks out as a safety precaution and will not restart until the underlying issue is resolved.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty NTC thermistor (flow or return sensor) Common
The NTC thermistors are small temperature-sensing components clipped onto the flow and return pipework. Over time they can degrade, drift out of calibration, or fail completely. When the PCB receives readings that are outside the expected range — or no reading at all — it triggers F56 and locks the boiler out. This is the most frequently identified root cause.
- Loose or damaged wiring to the sensors Common
The wiring harness connecting the thermistors to the PCB can become corroded, chafed, or simply work loose over years of heat cycling. Even a slightly intermittent connection is enough for the PCB to flag a sensor fault and lock out. Engineers often find this during a visual inspection before any components are swapped.
- Circulation fault — pump, valve, or blockage Common
If water is not circulating properly, the temperature difference between flow and return will behave abnormally, triggering F56. Common culprits are a seized or sluggish pump, a zone valve that has stuck closed, or a heavily sludged heat exchanger restricting flow. A system with no magnetic filter fitted is at greater risk of progressive sludge build-up.
- Flow and return sensors incorrectly fitted or swapped Sometimes
If sensors are fitted to the wrong pipes — or the flow and return pipework itself is crossed during installation or after recent work — the boiler will see temperature relationships that make no physical sense and lock out. This is more likely to appear after a recent boiler installation or plumbing work.
- Defective internal expansion vessel Sometimes
A waterlogged or under-pressurised expansion vessel can cause unusual pressure fluctuations during a heating cycle, which in turn affect circulation behaviour. The official Glow-Worm fault documentation lists this as a recognised trigger for F56, though it is less common than sensor or wiring faults.
- Limescale or sludge restricting the heat exchanger Sometimes
Heavy limescale or magnetite sludge narrows the waterways inside the heat exchanger, slowing flow to the point where temperatures spike abnormally. This is more common in hard-water areas and in older systems that have never been power-flushed or had inhibitor added.
How to fix it
- Reset the boiler once DIY safe
Glow-Worm boilers are typically reset by turning the central heating control knob to the reset position (marked with a power symbol), holding for a few seconds, then returning it to your desired temperature setting. Wait about 30 seconds before the boiler attempts to restart. A single reset is worth trying — an isolated glitch can occasionally clear this way. Do not repeatedly reset if the fault returns, as this can mask a developing problem.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top it up using the filling loop (usually a braided flexible hose with one or two valves beneath the boiler). Open the valves slowly until the gauge reaches around 1.2 bar, then close them fully. After topping up, try another reset. If pressure keeps dropping, there is a leak somewhere that an engineer must investigate.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If nothing gas-powered will light, contact your gas supplier — there may be a supply interruption. If gas supply is fine, the fault lies within the boiler or heating system.
- Do not attempt to access the boiler's internal components Gas Safe engineer
Glow-Worm explicitly advise against opening the boiler casing yourself. The sensors, wiring, pump, expansion vessel, and PCB are all internal components. Disturbing them without the correct training creates safety risks and will likely invalidate your warranty. Leave internal diagnosis to a qualified engineer.
- Have a Gas Safe engineer test the NTC thermistors Gas Safe engineer
An engineer will use a multimeter to measure the resistance of each thermistor at a known water temperature and compare the reading against the manufacturer's specification. A sensor that reads outside tolerance, or gives an open/short circuit, is confirmed faulty and replaced. Thermistor replacement is one of the quicker and more affordable fixes for F56.
- Have the engineer inspect wiring and sensor connections Gas Safe engineer
The engineer will check that all wiring plugs are fully seated, look for signs of corrosion or heat damage on the connectors, and test continuity along the harness. Damaged wiring sections are repaired or the loom is replaced as necessary.
- Have the engineer assess circulation — pump, valves, and pipework Gas Safe engineer
The engineer will check that the pump is running at the correct speed and producing adequate flow, that all zone valves are opening and closing correctly, and that flow and return sensors are clipped to the right pipes. If sludge is suspected, a system flush or full power-flush may be recommended.
- Have the engineer inspect and re-pressurise the expansion vessel if needed Gas Safe engineer
The engineer will check the charge pressure of the internal expansion vessel using a Schrader valve. If the diaphragm has failed or the vessel is waterlogged, it will need to be re-pressurised or replaced to restore stable system pressure during heating cycles.
- Call a Gas Safe registered engineer if the fault has not resolved Gas Safe engineer
If the reset and pressure checks have not cleared F56, the diagnosis requires professional equipment and Gas Safe competence. Book an engineer promptly — continued lockouts mean no heating or hot water, and leaving a faulty expansion vessel or circulation problem unaddressed can lead to further damage.
Parts you may need
- NTC flow thermistor · from £18
- NTC return thermistor · from £18
- Wiring loom / sensor harness · from £45
- Central heating pump · from £120
- Internal expansion vessel · from £75
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £110–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F56 fault myself?
The only steps a homeowner should take are a single boiler reset, checking and topping up system pressure, and confirming the gas supply is on. Everything else — testing thermistors, inspecting wiring, checking the pump, or accessing any internal part — must be done by a Gas Safe registered engineer. Opening the boiler casing yourself is unsafe and will void your Glow-Worm warranty.
How much does it typically cost to repair a Glow-Worm F56 fault in the UK?
For the most common repairs — replacing one or both NTC thermistors, or fixing a wiring fault — you should expect to pay around £110 to £200 all-in including labour. A pump replacement typically runs £200 to £380. If the engineer finds that a full power-flush is needed to clear sludge, or that the PCB has failed, costs can reach £400 to £600 or more. It is worth getting a quote before work begins, as a PCB or heat exchanger repair on an older boiler may make a boiler replacement a more sensible investment.
Why does F56 keep coming back after I reset my Glow-Worm boiler?
A recurring F56 after reset almost always means the underlying fault has not been fixed. Thermistors that are marginal, wiring that is intermittently losing contact, a pump that is slowing down, or a partially blocked heat exchanger can all allow the boiler to restart briefly before locking out again. Repeated resets are not a solution and can mask a worsening problem. If F56 returns after one or two resets, book an engineer rather than continuing to reset.
Could low boiler pressure cause an F56 fault code?
Indirectly, yes. Very low system pressure reduces circulation efficiency, which can cause abnormal temperature behaviour that triggers a lockout. Topping the system up to around 1.2 bar via the filling loop is a sensible first step. However, if the pressure is consistently dropping, that suggests a leak or a failed expansion vessel — both of which need a Gas Safe engineer to investigate and fix properly.