Glow-worm F71 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F71 fault code mean?
The F71 code appears on Glow-Worm boilers when the control board detects a problem with one or both of the water temperature sensors — specifically the flow sensor (which monitors water leaving the boiler) or the return sensor (which monitors cooler water coming back from the heating circuit). These sensors are NTC thermistors: small resistive components whose electrical resistance changes as temperature rises or falls, giving the PCB a continuous read-out of water temperature. When the PCB receives a reading that is implausible, out of range, or missing entirely, it triggers F71 and shuts the boiler down as a precaution. The fault can point to a wiring or connector issue, a failed thermistor, a circulation problem, or — less commonly — a defective internal expansion vessel or PCB fault affecting how sensor signals are interpreted.
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 electrical connection to flow or return sensor Common
Over time, the wiring harness connectors that plug into the NTC thermistors can loosen, corrode, or suffer insulation damage. A poor connection causes erratic or absent resistance readings, which the PCB interprets as a sensor fault and triggers F71. This is the first thing a Gas Safe engineer will check because it is also the quickest to fix.
- Failed NTC thermistor (flow or return) Common
The thermistor itself can degrade or fail outright, especially in older boilers or those with contaminated system water. When the component's resistance drifts outside the expected range — or goes open-circuit — the PCB has no reliable temperature data and locks out with F71. Replacing the sensor is a straightforward job for an engineer and the parts are relatively inexpensive.
- Circulation fault (pump or blocked system) Sometimes
If hot water is not moving around the circuit properly — due to a failing pump, a stuck zone valve, or sludge build-up — the temperature differential between flow and return can become abnormal, confusing the PCB into raising a sensor-related fault. This is why Glow-Worm's own guidance flags circulation issues as a possible root cause of F71.
- Defective internal expansion vessel Rare
A waterlogged or under-pressurised expansion vessel can cause pressure fluctuations that affect boiler operation and, in some cases, contribute to sensor readings that trigger fault codes. This is usually identified only after the more obvious electrical and sensor causes have been ruled out.
- PCB fault affecting sensor signal processing Rare
In a small number of cases the PCB itself is unable to correctly read the sensor signals even when the sensors and wiring are intact. This is relatively uncommon and is typically diagnosed by an engineer only after testing has confirmed the sensors and connections are fine.
How to fix it
- Check your gas supply is working DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If you have no gas at all, contact your gas supplier before doing anything else — this is not a boiler fault in that case.
- Attempt a single boiler reset DIY safe
On most Glow-Worm models, rotate the CH control knob to the reset position (marked with a power symbol), hold for a few seconds, then wait around 30 seconds before turning it back to your desired temperature setting. If the boiler fires up and runs normally, monitor it over the next hour or so. Do not reset more than once or twice — repeated resets without a successful outcome will not fix an underlying fault and may mask a developing problem.
- Check system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the heating is cold. If it has dropped below 1 bar, you can top it up using the filling loop (refer to your boiler's user manual for the exact procedure). Low pressure alone is unlikely to cause F71, but it is worth confirming the system is in good shape before calling an engineer.
- Engineer tests sensor wiring and connectors Gas Safe engineer
A Gas Safe registered engineer will open the boiler casing and use a multimeter to check the wiring loom and connector pins for the flow and return sensors. Damaged insulation, corroded pins, or a loose connector are common findings that can often be repaired or replaced at relatively low cost.
- Engineer measures thermistor resistance values Gas Safe engineer
Using a multimeter, the engineer will measure the electrical resistance of each NTC thermistor at a known temperature and compare the result against the manufacturer's specification curve. A sensor reading outside tolerance will be replaced. The parts themselves are inexpensive and the job is usually completed in a single visit.
- Engineer inspects the circulation system Gas Safe engineer
If the sensors and wiring test as healthy, the engineer will assess pump operation and check for blockages or stuck valves that could be causing abnormal temperature differentials across the heat exchanger. A power flush or pump replacement may be recommended at this stage.
- Engineer checks the expansion vessel Gas Safe engineer
If circulation is also fine, the engineer will test the expansion vessel charge pressure and inspect it for waterlogging. A re-pressurisation or replacement may be required.
- Engineer investigates PCB if all other causes are ruled out Gas Safe engineer
As a last resort, the PCB may be tested or replaced. This is a more expensive repair and a good engineer will only recommend it once sensors, wiring, circulation, and the expansion vessel have all been eliminated as the root cause.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler does not stay running after a reset, or if it trips F71 again shortly afterwards, call a Gas Safe registered engineer to diagnose and repair the fault. Do not continue resetting the boiler repeatedly. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- NTC flow temperature sensor (thermistor) · from £18
- NTC return temperature sensor (thermistor) · from £18
- Sensor wiring harness / connector loom · from £35
- Central heating pump · from £75
- Internal expansion vessel · from £55
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 the Glow-Worm F71 fault dangerous?
The boiler's decision to lock out when it detects an F71 is itself a safety measure — it is preventing the boiler from operating without reliable temperature data, which could lead to overheating. While the fault code itself is not an immediate safety emergency like a gas leak, you should not attempt to bypass or repeatedly reset the boiler. Get the fault diagnosed by a Gas Safe registered engineer as soon as conveniently possible, and sooner still if you have no heating or hot water in cold weather.
Can I fix the Glow-Worm F71 fault myself?
A single reset is the only action a homeowner should attempt. Everything beyond that — opening the boiler casing, testing sensors, inspecting wiring, or working on the pump — must be carried out by a Gas Safe registered engineer. NTC sensors are inexpensive parts, but the diagnostic process requires a multimeter and knowledge of the boiler's resistance specifications, and working inside the boiler casing without Gas Safe registration is both unsafe and illegal for gas work.
How much does it typically cost to repair a Glow-Worm F71 fault?
Most F71 repairs fall in the range of £120 to £380 including callout, labour, and parts. A straightforward sensor replacement or wiring repair at the lower end typically costs £120–£200. If the fault turns out to involve the pump or expansion vessel the bill is more likely to sit around £250–£380. PCB replacement is a more significant expense and can push costs higher, but this is an uncommon outcome for an F71 — your engineer should exhaust cheaper explanations first. Prices in London and the South East tend to run 20–35% higher than elsewhere in the UK.
Why does my Glow-Worm keep showing F71 after I reset it?
If F71 returns within minutes or hours of a reset, the underlying fault has not been resolved. Repeated resetting will not fix a failed sensor, damaged wiring, or a circulation problem — it will only delay the repair. Each time the boiler relocks, it is telling you the PCB is still receiving an unacceptable reading from the temperature sensor circuit. Call a Gas Safe engineer to run proper diagnostic checks rather than continuing to reset.