Glow-worm F85 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F85 fault code mean?
The F85 code appears on your Glow Worm boiler when the PCB (printed circuit board) has detected a problem with one or both of the NTC thermistors — the temperature sensors fitted to the flow and return heating pipes. The flow thermistor sits on the pipe carrying hot water out of the boiler, while the return thermistor monitors water coming back in after circulating around the heating system. These sensors continuously send resistance readings to the PCB, which uses them to control heat output. When a reading falls outside the expected range — due to a failed sensor, a wiring fault, or in rarer cases a PCB issue — the boiler locks out and displays F85 to prevent unsafe operation. A simple reset is unlikely to clear this fault permanently; the underlying sensor or wiring problem needs to be diagnosed and resolved by a Gas Safe registered engineer.
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
- Failed or out-of-range NTC thermistor Common
One of the flow or return temperature sensors has drifted out of calibration or failed entirely. An engineer can measure the thermistor's resistance at a known temperature and compare it against Glow Worm's reference charts. If the reading is wrong, the sensor needs replacing — this is the most frequent cause of F85.
- Loose or disconnected sensor plug Common
The plug connecting an NTC thermistor to the wiring harness can work loose over time due to everyday vibration from the fan, pump, and burner. A partially seated connector gives the PCB an intermittent or absent signal, which it interprets as a sensor fault even if the thermistor itself is perfectly healthy.
- Damaged or corroded wiring harness Sometimes
The cables running from the thermistors back to the PCB can suffer chafing, heat damage, or corrosion — especially if there has been a minor water leak near the sensor pocket. Breaks or high resistance in the circuit mimic a faulty sensor from the PCB's point of view. An engineer will check continuity throughout the wiring run.
- Water ingress around the sensor pocket Sometimes
A small leak at the sensor housing or a nearby joint can allow water to track along the wiring and corrode the connector pins or the sensor itself. This is more common on older installations and can damage both the thermistor and the surrounding wiring simultaneously.
- Faulty PCB Rare
If the thermistors and all associated wiring test correctly, the PCB input circuit itself may have failed. The PCB is responsible for sending a reference voltage to each sensor and interpreting the returned signal, so a faulty input channel produces the same lockout as a bad sensor. PCB failure is a less common root cause but must be considered once everything else has been ruled out.
How to fix it
- Check whether the boiler has locked out or is showing a continuous fault DIY safe
Look at the display — F85 will typically be accompanied by a lockout indicator. Note whether the fault appeared suddenly or has been intermittent, as this helps the engineer narrow down the cause (intermittent faults often point to a loose connector rather than a fully failed sensor).
- Do not repeatedly reset the boiler DIY safe
The reset function is intended to be used after a fault has been repaired, not as a way to clear an active fault. Forcing the boiler to run with a sensor fault risks masking real problems and could allow components to overheat. Avoid resetting more than once or twice while waiting for an engineer.
- Check the boiler's system pressure DIY safe
Glance at the pressure gauge. If it reads below 1 bar, low pressure can contribute to abnormal temperature sensor readings. Top up via the filling loop to restore pressure to between 1 and 1.5 bar following your boiler's user guide. This is unlikely to clear F85 on its own, but rules out a secondary issue.
- Visually inspect accessible pipework for leaks near the sensors DIY safe
Without opening the boiler casing, check for any damp patches, staining, or dripping around the lower pipework visible beneath the boiler. Report any findings to the engineer — water tracking along the wiring harness is a known cause of this fault.
- Engineer: inspect sensor plugs and wiring harness Gas Safe engineer
A Gas Safe engineer will open the boiler casing and visually inspect the flow and return NTC thermistors, checking that both connector plugs are fully seated, that there is no corrosion on the pins, and that the wiring shows no signs of chafing, heat damage, or moisture ingress.
- Engineer: test thermistor resistance with a multimeter Gas Safe engineer
The engineer measures the resistance of each NTC thermistor at the ambient temperature of the boiler room and compares the result against Glow Worm's published reference values. A reading outside tolerance confirms the sensor has failed and needs to be replaced. Wiring continuity back to the PCB is tested at the same time.
- Engineer: replace the faulty NTC thermistor Gas Safe engineer
If one or both sensors test out of range, the engineer will fit a new manufacturer-approved NTC thermistor, ensuring the sensor seats correctly in its pocket and the connector locks firmly into place. The boiler is then restarted and monitored through a full heating cycle to confirm the fault has cleared.
- Engineer: inspect and test the PCB if sensors and wiring are healthy Gas Safe engineer
Should both thermistors and the wiring harness test correctly, the PCB input circuit becomes the suspect. The engineer will assess whether the board can be repaired or whether a replacement PCB is required. Once any parts are fitted, the boiler is reset and tested.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
F85 is a sensor lockout that almost always requires professional diagnosis and repair. Contact a Gas Safe registered engineer — you can verify registration at gassaferegister.co.uk — and describe the fault code along with any observations about when it first appeared or how often it recurs.
Parts you may need
- Flow NTC thermistor (Glow Worm compatible) · from £18
- Return NTC thermistor (Glow Worm compatible) · from £18
- Wiring harness / sensor loom · from £45
- Replacement PCB · 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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I reset my Glow Worm boiler to clear the F85 code?
A single reset is worth trying, but do not rely on it. F85 is a protective lockout triggered when the PCB detects a sensor reading outside safe limits. If a reset clears it temporarily but the code returns, the underlying fault — a failed thermistor, a loose connector, or damaged wiring — is still present and still needs a Gas Safe engineer to fix properly. Repeatedly forcing the boiler back on without a repair risks component damage.
How much does it cost to fix a Glow Worm F85 fault?
For the most common repairs — replacing one or both NTC thermistors and any associated wiring — you can typically expect to pay between £120 and £320 including parts and labour. Wiring-only repairs tend to sit at the lower end of that range. If the PCB turns out to be at fault, the cost rises considerably: a replacement PCB for a Glow Worm can cost £350–£500 or more once fitted, which is why engineers always confirm the sensors and wiring are healthy before condemning the board.
Which Glow Worm boilers show the F85 code?
F85 is associated with the Glow Worm Betacom, Ultracom2 cxi, Ultracom2 sxi, and Ultracom2 35 model ranges. If you own a different Glow Worm model and are seeing a similar fault, the code format may differ — check your boiler's user manual or call a Gas Safe engineer to confirm the correct interpretation.
Why do NTC thermistors fail on Glow Worm boilers?
NTC thermistors are relatively long-lived components, but they can fail for several reasons: gradual degradation after years of thermal cycling, physical damage from a nearby leak, corrosion of the connector pins, or vibration from the pump and fan gradually loosening the wiring. Boilers that miss their annual service are more likely to develop these faults undetected, since a service includes checking sensor readings and connections. Keeping up with yearly Gas Safe servicing is the best way to catch a deteriorating thermistor before it causes a lockout.