Glow-worm Temperature 40 Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm Temperature 40 Flashing fault code mean?
On older Glow-Worm boilers that use a row of temperature indicator lights (typically 30, 40, 50, 60, 70 °C) rather than an alphanumeric display, the '40' light flashing can mean two very different things. During normal operation the light may blink briefly when the burner cycles off after reaching the set temperature — this is expected behaviour and nothing to worry about. However, if the '40' light flashes persistently, the boiler fails to heat properly, or it keeps cutting out, it is signalling that the boiler cannot progress beyond a low flow temperature. This points to an underlying problem such as restricted circulation, a faulty temperature sensor, a struggling pump, or sludge in the heat exchanger. Models commonly affected include certain Glow-Worm Compact, SXI, and older Combi ranges. Because Glow-Worm shares platform technology with Vaillant, the underlying fault logic is closely related to Vaillant error codes such as F13 (temperature sensor fault), F20 (overheat), and F5 (overheat/airlock).
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
- Poor circulation or restricted water flow Common
If the boiler cannot push enough hot water around the system, the heat exchanger temperature differential becomes excessive and the boiler drops into a waiting state at a low temperature. This can be caused by an undersized or failing pump, partially closed valves, or thermostatic radiator valves shutting off too many circuits simultaneously — particularly common in summer when most TRVs are closed.
- Faulty NTC thermistor or temperature sensor Common
The boiler relies on NTC thermistors to read flow and return temperatures. A failing sensor (analogous to the F13 or F15 fault on digital Glow-Worm models) can cause the boiler to misread its own temperature, making it appear stuck at a low reading and triggering the 40 °C indicator to flash as though the boiler cannot warm up.
- Sludge or scale build-up in the heat exchanger Common
Magnetite sludge or limescale deposits inside the heat exchanger restrict water flow and cause uneven heating. The boiler may short-cycle or fail to reach its set temperature, leaving the low-temperature indicator flashing. Kettling noises often accompany this fault.
- Seized or failing circulator pump Sometimes
A pump that has seized or is running below specification cannot move water efficiently. Hot water stagnates in the boiler rather than circulating, meaning the boiler never fully warms the system and may cut out on the low-temperature indicator.
- Airlocks in the central heating system Sometimes
Air trapped in the pipework or heat exchanger interrupts water flow. This can follow recent system work, a pressure loss event, or bleeding radiators without properly re-pressurising. Airlocks cause short-cycling and can prevent the boiler from progressing beyond a low temperature.
- Overheat thermostat tripped Sometimes
On older Glow-Worm models without a digital display, a tripped overheat stat (equivalent to the F20 fault on digital models) can prevent the burner from firing up to full temperature. The boiler may appear stuck at a low temperature even though the underlying cause is a safety device that has activated due to a previous overheating event.
How to fix it
- Check whether the flashing is normal operation DIY safe
Look at the boiler's behaviour overall. If the '40' light flashes briefly after the boiler has been running and the radiators feel warm, this is likely the burner cycling off at the set temperature — perfectly normal. Only investigate further if the boiler is not heating properly, if the light flashes constantly, or if the boiler keeps cutting out.
- Verify the system pressure is between 1.0 and 1.5 bar DIY safe
Find the pressure gauge on the boiler (usually on the front panel). If it reads below 1 bar, top up via the filling loop until the needle sits between 1.0 and 1.5 bar, then tighten the filling loop valves. Low pressure impairs circulation and can cause the boiler to behave erratically.
- Bleed the radiators to release trapped air DIY safe
Start with the radiator closest to the boiler and work outwards. Use a radiator bleed key to open the bleed valve until water (not air) runs steadily, then close it. After bleeding all radiators, re-check and top up system pressure if it has dropped. This removes airlocks that restrict water flow.
- Check that all radiator and system valves are fully open DIY safe
Make sure thermostatic radiator valves, lockshield valves, and any zone valves are open enough to allow adequate flow. If nearly all TRVs are closed (common in summer), open one or two manually to ensure a minimum flow circuit through the boiler.
- Reset the boiler and observe DIY safe
On most affected Glow-Worm models, turn the CH control knob to the reset position (marked with a power symbol), hold for a moment, then turn back to your desired temperature setting. Wait to see whether the boiler fires and progresses past 40 °C. Repeat no more than two or three times — if the fault returns, stop resetting and move to the next step.
- Have a Gas Safe engineer inspect the NTC thermistors and wiring Gas Safe engineer
A qualified engineer will test the flow and return temperature sensors with a multimeter. A sensor reading outside its expected resistance range must be replaced. Wiring connections to the PCB should also be checked for corrosion or looseness, as these can cause false low-temperature readings.
- Have a Gas Safe engineer assess the circulator pump Gas Safe engineer
The engineer will check pump speed settings, test for correct output, and inspect for seizure. In most cases, replacing a failing pump is more reliable than attempting a repair. The engineer will also confirm the pump is correctly sized for the system.
- Have a Gas Safe engineer check the overheat thermostat Gas Safe engineer
If the overheat stat has tripped, the engineer will identify and rectify the underlying cause before manually resetting the stat. Simply resetting it without finding the root cause risks repeat tripping and potential damage.
- Consider a power flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of magnetite or limescale — discoloured system water, cold spots on radiators, or kettling — a power flush will clear deposits from the heat exchanger and pipework. A chemical inhibitor should be added afterwards to slow future build-up. This is carried out by a heating engineer, not necessarily a Gas Safe engineer, though many are both.
- Call a Gas Safe registered engineer if any fault persists Gas Safe engineer
If the '40' light continues to flash after the homeowner checks above, or if you are at all unsure about the boiler's condition, contact a Gas Safe registered engineer. You can verify registration at gassaferegister.co.uk. Do not continue running the boiler if it is repeatedly cutting out, as this could indicate an underlying safety issue.
Parts you may need
- NTC thermistor / flow temperature sensor · from £25
- Return temperature sensor (NTC) · from £25
- Overheat thermostat (high-limit stat) · from £20
- Circulator pump (compatible replacement) · from £85
- PCB (printed circuit board) · from £180
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–£400, depending on the underlying cause.
Frequently asked questions
Is the Temperature 40 light flashing always a fault on my Glow-Worm boiler?
Not necessarily. On older Glow-Worm models with analogue temperature indicator lights, the '40' light can flash briefly as a normal part of the burner cycling off once the set temperature is reached. This is especially common in summer when fewer radiators are calling for heat. It becomes a concern if the flashing is continuous, the boiler is not heating the home properly, or it keeps cutting out — in those cases, one of the faults described on this page is likely the cause.
Can I fix the Temperature 40 flashing fault myself, or do I need an engineer?
Some preliminary checks are safe for a homeowner: topping up system pressure, bleeding radiators, checking valves are open, and resetting the boiler. If those steps do not resolve the issue, you will need a Gas Safe registered engineer. Replacing sensors, testing the pump, resetting the overheat thermostat, or working on any gas component must only be done by a qualified professional.
How much does it typically cost to fix this fault in the UK?
Most repairs for this fault fall in the range of £120–£400 including parts and labour. A sensor replacement is usually towards the lower end (£120–£200), while a pump replacement tends to sit in the £200–£400 range. A power flush, if sludge is the root cause, is typically an additional £300–£600 on top. In rare cases where the PCB has failed, costs can rise to £350–£600 or more — worth knowing before committing to a repair on an older boiler.
My Glow-Worm boiler does not show F-codes — how do I know what is wrong?
Older Glow-Worm models use rows of LED or temperature indicator lights rather than an alphanumeric display. The flashing pattern of those lights serves the same purpose as F-codes on newer models. The '40' light flashing persistently broadly corresponds to the F13 (temperature sensor fault) or F20 (overheat) logic used on more modern Glow-Worm and Vaillant boilers. A Gas Safe engineer familiar with older Glow-Worm ranges will know how to interpret these indicator patterns and diagnose the underlying cause.