Glow-worm F43 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F43 fault code mean?
The F43 code appears on Glow-Worm boilers when the PCB (Printed Circuit Board) detects that something is outside of normal operating parameters but cannot identify the precise source of the problem. Think of it as the boiler's way of saying 'something isn't right, but I'm not sure what.' The PCB continuously monitors readings from temperature and pressure sensors across the system; when any of those readings fall outside acceptable limits — or a sensor stops communicating altogether — the boiler locks out and displays F43 to prevent further damage. Because the code is deliberately broad, the range of possible root causes is wider than with most fault codes, and proper diagnosis by a Gas Safe registered engineer is essential to identify and fix the actual underlying issue.
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
- PCB fault or failure Common
Because the F43 is a catch-all code, a faulty PCB is the most frequently identified culprit. The PCB coordinates signals between every electrical component in the boiler; even a minor fault on the board — caused by a power surge, a small internal leak dripping onto the board, or age-related degradation — can disrupt those signals enough to trigger a generic lockout. A Gas Safe engineer can use a multimeter to check whether the board is receiving and distributing power correctly.
- Water circulation or pump problem Common
Engineers attending F43 faults on Glow-Worm Flexicom boilers have regularly found the circulation pump to be the root cause. A sluggish or seized pump means hot water is not moving through the system at the expected rate, causing temperature sensors to report unusual readings. The PCB interprets this as an unspecified fault and shuts the boiler down.
- Sludge build-up in the heat exchanger or system Common
Over time, magnetite sludge and scale can accumulate in the plate heat exchanger and pipework, restricting flow and causing abnormal pressure or temperature readings. Systems that have never been power-flushed or had inhibitor added are particularly vulnerable. This is an indirect trigger — the sludge causes circulation or heat-transfer problems that the PCB then flags as a generic error.
- Loose wiring or corroded connections Sometimes
Vibration, age, or accidental disturbance during a previous service can leave wiring connectors loose or contacts corroded. A poor connection means sensors or components send intermittent or no signal to the PCB, which can produce a generic fault rather than a component-specific code. Engineers often discover this quickly on visual inspection inside the casing.
- Diverter valve fault Sometimes
A sticking or worn diverter valve can disrupt the balance of water flow between the central heating and hot water circuits. The resulting abnormal flow pattern may be detected by the PCB as an out-of-tolerance condition, leading to an F43 lockout, particularly if it also causes pressure fluctuations within the system.
- Intermittent cold-weather start-up issue Rare
Some Glow-Worm owners — particularly those with Flexicom models — have reported F43 appearing only on cold mornings, with the boiler firing normally after a single reset. This is likely linked to marginal sensor readings or sluggish circulation when the system is very cold. While a reset may clear it temporarily, recurring cold-start F43 faults should still be investigated by an engineer to rule out a deteriorating component.
How to fix it
- Check that your gas supply is working DIY safe
Before assuming a boiler fault, confirm that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply, contact your gas supplier rather than investigating the boiler further.
- Reset the boiler once or twice DIY safe
Locate the reset button on your Glow-Worm boiler (usually marked with a flame symbol or labelled 'Reset') and hold it for the duration specified in your user manual — typically around 3 seconds. Allow the boiler a full minute to attempt a restart. If the F43 clears and the boiler runs normally, monitor it closely. Do not reset more than two or three times in total; repeated resets without a fix can mask a serious underlying fault.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler — for most Glow-Worm models, the correct cold pressure is between 1.0 and 1.5 bar. If it reads below 0.8 bar, you can top it up using the filling loop (a silver braided hose beneath the boiler). Open both valves slowly until the gauge reads around 1.2 bar, then close the valves and attempt a reset. If pressure is already normal, move on.
- Note whether the fault is intermittent or persistent DIY safe
Before calling an engineer, make a note of when the F43 appears — for example, only on cold mornings, during hot water demand, or constantly. This information helps the engineer narrow down the diagnosis and can reduce call-out time.
- Do not open the boiler casing or inspect internal components Gas Safe engineer
The F43 code requires internal fault-finding that goes beyond homeowner checks. Do not remove the boiler casing, inspect wiring, test the PCB, or attempt to adjust any internal components. This work must only be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
Because F43 is a generic code, a Gas Safe engineer will need to carry out a structured fault-finding process — checking PCB power supply and signals, inspecting wiring and connections, testing the pump, and assessing the system for sludge. Only once the root cause is identified can the appropriate repair (PCB replacement, power flush, pump replacement, or wiring repair) be carried out safely.
Parts you may need
- Boiler PCB (Printed Circuit Board) · from £120
- Circulation pump · from £75
- Diverter valve · from £55
- Wiring harness · from £45
- Thermistor / temperature sensor · from £20
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F43 code dangerous?
The F43 itself is not an immediate safety emergency — the boiler has locked out as a precaution rather than continuing to run with an unidentified problem. However, because the code is generic, you should not repeatedly reset the boiler without getting a professional diagnosis. If you smell gas at any point, do not reset the boiler; leave the property and call the National Gas Emergency Service on 0800 111 999.
Can I fix the F43 fault myself?
Homeowner checks are limited to confirming the gas supply is on, topping up system pressure if it is low, and attempting one or two resets. Beyond that, F43 requires a Gas Safe registered engineer to diagnose. The most common causes — a faulty PCB, a failing pump, or sludge build-up — all involve either electrical testing inside the boiler or physical work on the heating system, neither of which a homeowner should undertake.
How much does it typically cost to fix a Glow-Worm F43 fault?
Most people pay between £150 and £400 to resolve an F43 fault. A straightforward wiring repair or pump replacement tends to sit at the lower end, while a full PCB replacement typically costs £220–£350 fitted. A system power flush, if sludge is identified as the cause, usually adds £300–£500. In rare cases where a PCB replacement runs to £500 or more and the boiler is old or out of warranty, it may be more economical to replace the boiler entirely — new installations typically cost £1,500–£2,800.
Why does my Glow-Worm show F43 only on cold mornings?
Intermittent cold-start F43 faults are occasionally reported on the Flexicom range. The most likely explanations are that a marginal component — such as a pump, sensor, or electrical connection — performs just within tolerance when warm but drifts outside acceptable limits when cold. A single successful reset does not mean the fault has resolved itself; if it happens more than once or twice, book an engineer to investigate before the fault becomes persistent or causes further damage.