Glow-worm F.71 fault codewhat it means and how to fix it
The flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.
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Get a Gas Safe registered engineer to look at your Glow-worm F.71
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What F.71 means
F.71 is the code for a flow sensor that passes every electrical test and still lies. Glow-worm's Energy7 and Ultimate3 manuals describe it as the "flow temperature sensor signalling constant value" with two causes: the sensor is "incorrectly positioned on flow pipe" or it is "defective". The Compact manual's three measures are, in order, check the positioning, check the positioning, then replace the sensor — which tells you where Glow-worm expect the fault to be.
The two-digit Ultracom2, Betacom 24a/28a and Betacom2 list F71 as "flow heating temperature sensor fault — unclipped or defective (no temperature variation)", and their remedy is the plainest sentence in the manual: "make sure that the sensor is clipped to the tube". The thermistor is a clip-on part, and a clip disturbed during a service or a heat exchanger change leaves it hanging in the casing air, reading room temperature while the water behind it climbs.
The board sees the burner firing and the number not moving and stops before the overheat protection has to. The Ultracom, Flexicom and Ultrapower tables carry no 71; on the CI range the nearest code is 19, "central heating NTC not connected to pipe", which is the same fault under a different number.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor clip off the pipe | Glow-worm's first and second measures are both "check the positioning". The clip springs off during work on the boiler and the sensor reads the casing air, which does not change when the burner lights. | Common |
| Sensor refitted on the wrong pipe | After a heat exchanger or pipe change the flow sensor can end up on a pipe that does not warm with the burner. The reading is real, just not of the flow. | Occasional |
| Thermistor drifted to a fixed value | "Flow temperature sensor defective" — a bead that has aged into a near-constant resistance. Proven by warming the sensor and watching the reading fail to follow, and replaced. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
30 minutes
Often no part at all — a sensor clipped back on within a call-out fee of £80–£130. A new thermistor £100–£160 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm flow or return temperature sensor (clip-on NTC)
£12–£35Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.00Get it checkedThe flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.Related: temperature sensors
- F.10Get it checkedF.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Related: temperature sensors
- F.11Get it checkedOn Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Related: temperature sensors
- F.13Get it checkedOn a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.Related: temperature sensors
- F.84Get it checkedThe return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.Related: temperature sensors
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.71
The boiler fires, runs a minute and shows F.71. Why?
That is the fault exactly. The board expects the flow reading to rise as the burner runs; when it does not, it assumes the sensor is not seeing the water and stops to protect the heat exchanger.
Did the last service cause it?
It is the most common explanation. The flow sensor clip is easy to disturb, and F.71 within days of a service or a repair is worth mentioning to whoever did the work.
Is this the same as F.83?
Related. F.83 is the board seeing no temperature change on either sensor and blaming low water; F.71 is the flow sensor alone signalling a constant value. Both often trace to a sensor off its pipe.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.