Glow-worm F83 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F83 fault code mean?
The F83 code means your Glow-Worm boiler has locked out because it detected little or no temperature increase when the burner fired. When the boiler starts a heating or hot water cycle, it expects to see the flow and return water temperatures rise within a set time. If those sensors report back nothing — or barely any change — the boiler shuts down as a safety measure. This usually points to one of three root causes: insufficient water in the system, a flow or return temperature sensor that is faulty or poorly positioned on the pipe, or a problem with the boiler's electronics. Until the underlying issue is sorted and the lockout is cleared, the boiler will not run.
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
- Low system pressure Common
If the system pressure has dropped below around 1 bar, there may not be enough water circulating for the sensors to register a meaningful temperature rise when the burner fires. This is the most frequent trigger for F83 and is straightforward to resolve by topping up via the filling loop.
- Flow or return NTC temperature sensor fault Common
The thermistors that measure flow and return water temperatures can drift out of calibration, develop a loose connection, or sit slightly off the pipe. Any of these will cause inaccurate readings, making the boiler believe no heating is occurring even when it is.
- Blocked or frozen condensate pipe Sometimes
A blocked or frozen condensate pipe can cause the boiler to shut down and produce secondary fault codes. If temperatures have been below freezing or the condensate pipe runs outside or through an unheated space, check here before assuming a more serious fault.
- Blocked burner jet Sometimes
Over time, carbon deposits can partially or fully block the small jet inside the burner. A restricted jet means gas flow is unsteady and the flame is weak, so the temperature rise is too small for the sensors to register a normal response.
- Sticking or faulty gas valve Sometimes
If the gas valve is not opening fully or is sticking, insufficient gas reaches the burner. The result is a weak flame or no flame at all, with no meaningful temperature increase — triggering the F83 lockout.
- PCB or internal communication fault Rare
The printed circuit board coordinates all the boiler's components. If it is damaged, or if wiring between the PCB and the sensors has become loose or corroded, the control system may misread or fail to receive temperature data entirely, causing a false F83.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should sit between 1 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop to top up the pressure until the needle reaches around 1.2–1.5 bar, then close the filling loop valves fully.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — a gas hob, for example — are working normally. If they are not, contact your gas supplier rather than continuing to reset the boiler.
- Check for a frozen or blocked condensate pipe DIY safe
If the weather has been below freezing, find the white plastic condensate pipe that runs from the boiler to an outside drain. If it feels solid or there is a gurgling noise from the boiler, the pipe may be frozen. You can carefully pour warm (not boiling) water over the frozen section to thaw it, or wrap it with a warm cloth. Never use a naked flame.
- Reset the boiler DIY safe
Once you have addressed any pressure or condensate issue, press and hold the reset button (refer to your model's manual for the exact location — on many Glow-Worm boilers it is clearly labelled on the fascia). Hold for around 3 seconds if prompted. Allow the boiler a full start-up attempt. Only reset once or twice — if the F83 returns immediately, stop resetting and call an engineer, as repeated resets without fixing the root cause can cause further damage.
- Engineer: inspect and test the flow and return temperature sensors Gas Safe engineer
A Gas Safe engineer should check that both NTC thermistors are firmly clipped to the correct pipe positions, test their resistance values against the manufacturer's specifications, and replace either sensor if it is reading outside tolerance.
- Engineer: inspect and clean the burner and jet Gas Safe engineer
The engineer should remove and inspect the burner assembly for carbon build-up, cleaning or replacing the burner jet as necessary. This restores a stable flame and allows a proper temperature rise to be detected.
- Engineer: check the gas valve and gas rate Gas Safe engineer
Using a manometer, a Gas Safe engineer can verify that gas inlet pressure and the boiler's gas rate are within the manufacturer's specified range. If the gas valve is sticking or not opening correctly, it can sometimes be freed; if it has failed, it will need replacing.
- Engineer: check wiring, connections, and PCB Gas Safe engineer
If all physical components test within spec, the engineer should examine the wiring loom for damaged insulation, loose terminals, or corrosion at the sensor connectors and PCB headers. A faulty PCB will need to be replaced, and the engineer should carry out a full functional check afterwards.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the steps above have not resolved the F83 fault, or if you were unable to identify the cause yourself, contact a Gas Safe registered engineer. Provide them with the fault code and a description of any checks you have already carried out.
Parts you may need
- NTC flow temperature sensor · from £25
- NTC return temperature sensor · from £25
- Burner jet / injector · from £20
- Gas valve · from £120
- PCB (printed circuit board) · from £220
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F83 fault myself?
Some of the checks are safe to do yourself: top up the system pressure to around 1.2–1.5 bar using the filling loop, thaw a frozen condensate pipe with warm water, and reset the boiler once or twice. If the fault clears and does not return, you are done. However, if the F83 comes back after a reset, or the pressure was fine all along, the cause is likely a faulty sensor, blocked burner, gas valve issue, or PCB problem — all of which require a Gas Safe registered engineer.
How much does it cost to fix a Glow-Worm F83 fault?
For most people, the repair falls between £150 and £350 including parts and labour. A sensor replacement or burner clean sits at the lower end of that range, while a gas valve replacement typically comes in around £250–£300. PCB replacement is significantly more expensive — often around £400–£500 — and if your boiler is older, it may be worth weighing that cost against a new boiler rather than investing heavily in an ageing unit.
Why does my Glow-Worm boiler keep showing F83 after I reset it?
If the fault code returns quickly after every reset, the underlying problem has not been fixed. Repeated resets without addressing the root cause can stress boiler components and may void your warranty. Stop resetting once you have tried two or three times without success and book a Gas Safe engineer to carry out a proper diagnosis.
Could low boiler pressure really cause an F83 fault?
Yes — it is actually the most common cause. When system pressure is too low, water circulation is reduced, meaning the flow and return sensors do not detect the expected temperature rise after the burner fires. Simply topping up the pressure to the recommended 1–1.5 bar (cold) and resetting the boiler is enough to clear the fault in many cases. If pressure keeps dropping, you likely have a small leak somewhere in the system that a heating engineer should locate and repair.