Glow-worm F51 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F51 fault code mean?
The F51 code is not listed in any officially published Glow-worm fault code documentation, which means your boiler display may be showing a misread or model-specific code. Glow-worm boilers — part of the Vaillant Group — use alphanumeric fault codes, and some displays can be tricky to read, particularly on older models where segments can appear merged or truncated. The most likely explanations are that the display is actually showing F5 (an overheat lockout), F61 (an electronic or PCB-related fault), or in some cases F.5 and F.1 appearing in quick succession. This page covers all three possibilities so that whatever you are seeing on your boiler, you can find the relevant guidance here. If your boiler is in lockout and you are unsure of the exact code, do not attempt to reset it repeatedly — read the steps below and, if in doubt, contact a Gas Safe registered engineer.
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
- Misread display showing F5 (overheat lockout) Common
The F5 fault is one of the most common Glow-worm codes and indicates the boiler has reached an unsafe operating temperature and shut itself down. This is frequently caused by airlocks in the heating system, a failing or seized circulation pump, or a build-up of sludge restricting water flow through the heat exchanger. The boiler display on some models can make F5 look like F51, especially at a glance.
- Misread display showing F61 (PCB or electronic fault) Common
F61 through F67 are documented Glow-worm electronic fault codes pointing to issues with the printed circuit board or related electronic components. A faulty PCB can cause the boiler to behave unpredictably and display codes that appear garbled or unfamiliar. If your display seems to be flickering or showing incomplete digits, a PCB fault may itself be responsible for the unusual reading.
- Model-specific or undocumented fault code Sometimes
Glow-worm has produced a wide range of boilers over the years, and some older or less common models carry fault codes that are not widely documented in publicly available resources. It is possible that F51 is a genuine code on your specific appliance that simply does not appear in general fault code listings. In this case, the boiler manual or a Glow-worm trained Gas Safe engineer with diagnostic equipment is the only reliable source of interpretation.
- Gas valve safety lockout (false flame detection) Rare
On Vaillant Group boilers, a false flame fault occurs when the boiler detects a flame signal at a time when it has not requested ignition — suggesting the gas valve may be failing to close fully. This is a serious safety issue. While documented under different code numbers on Glow-worm boilers (such as F26 or F77), a gas valve fault on an older or lesser-documented model could conceivably produce an unfamiliar code on the display.
How to fix it
- Read the display carefully and cross-reference with your boiler manual DIY safe
Before doing anything else, look at the display again in good lighting. On Glow-worm boilers, fault codes are usually shown as two characters. Check whether the code could be F5, F.5, F61, or two separate codes cycling on the display. Locate your boiler's installation or user manual — the fault code list is usually at the back — and match what you see to the listed codes. If you no longer have the manual, Glow-worm's website provides PDF downloads by model name.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (such as a gas hob or gas fire) are working normally. If they are not, the issue may be with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency line on 0800 111 999 if you suspect a supply problem or smell gas.
- Check and top up boiler pressure if low DIY safe
A low system pressure can contribute to overheat-type faults by reducing water flow through the heat exchanger. Check the pressure gauge on the front of your boiler — it should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop (usually a braided hose with two valves beneath the boiler) to slowly top up the pressure. Stop at 1.5 bar and close both valves firmly. Do not overpressurise the system.
- Attempt a single boiler reset DIY safe
On most Glow-worm models, the reset procedure involves turning the central heating control knob to the reset position (marked with a power or reset symbol), holding it there for a few seconds, then releasing and turning to your desired temperature setting. If the fault code clears and the boiler fires up normally, monitor it closely over the next hour. If the fault returns within a short period, do not keep resetting — repeated resets without fixing the root cause can mask a developing problem or cause additional wear.
- Thaw a frozen condensate pipe if applicable in cold weather DIY safe
If the fault has appeared during freezing temperatures, your condensate pipe — the white plastic pipe that exits the boiler and typically runs outside — may be frozen. This can cause a lockout on many modern condensing boilers. You can carefully pour warm (not boiling) water over the external section of the pipe to thaw it, then reset the boiler once. If this resolves the issue, consider lagging the pipe to prevent a recurrence.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
If the fault persists after a reset, if you cannot identify the code from your manual, or if you have any concern about a gas valve issue or PCB failure, you must call a Gas Safe registered engineer. They have the manufacturer-specific diagnostic tools to read live fault data, test components safely, and carry out any required repairs. Do not attempt to inspect or interfere with the gas valve, PCB, heat exchanger, or any internal gas-carrying components yourself — this is illegal without Gas Safe registration and potentially dangerous. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Circulation pump · from £80
- Gas valve · from £200
- PCB (printed circuit board) · from £230
- NTC thermistor (temperature sensor) · from £25
- Condensate pipe insulation lagging · from £8
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Is F51 a real Glow-worm fault code?
F51 does not appear in any publicly available Glow-worm fault code documentation from the manufacturer or Gas Safe industry sources. The most likely explanation is that the display is showing F5 (overheat) or F61 (electronic fault), which can look similar on some Glow-worm displays. If your boiler manual does not list F51, photograph the display and share it with a Gas Safe engineer who can connect diagnostic equipment and read the actual fault data directly from the boiler's electronics.
What does F5 mean on a Glow-worm boiler?
F5 is a documented Glow-worm fault indicating an overheat lockout — the boiler has detected that its heat exchanger or primary circuit has exceeded a safe temperature and has shut down as a precaution. Common causes include a blocked or sludged heat exchanger, an airlock in the heating system, or a failing circulation pump. A one-off occurrence can sometimes be resolved by bleeding radiators, checking system pressure, and resetting the boiler. However, if F5 keeps returning, a Gas Safe engineer should investigate further — often a power flush or pump replacement is needed.
What does F61 mean on a Glow-worm boiler and will it need a new PCB?
F61 (and codes F62–F67) on Glow-worm boilers points to an electronic fault, most commonly involving the PCB. However, not every F61 requires a full PCB replacement — sometimes the fault is caused by a wiring connection that has corroded or come loose, a faulty sensor sending bad signals to the board, or a component on the PCB that can be repaired rather than replaced. A Gas Safe engineer will test the board and associated wiring before recommending a replacement. A new PCB typically costs £150–£300 for the part, with labour on top, so total costs of £300–£500 are common. In rare cases where the board is no longer available, or the boiler is old, a full boiler replacement may be more economical.
Should I keep resetting my Glow-worm boiler if the fault code keeps coming back?
No — resetting is only appropriate as a first response, not a repeated workaround. If the fault code returns after one or two resets, there is an underlying issue that needs professional diagnosis. Repeatedly resetting a boiler with a genuine fault can in some cases cause further damage to components, and in the case of gas valve faults, could pose a safety risk. If the boiler is going into lockout regularly, book a Gas Safe engineer rather than continuing to reset.