Glow-worm Temperature 90 Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm Temperature 90 Flashing fault code mean?
On the Glow Worm Betacom c range, the boiler does not use alphanumeric fault codes in the conventional sense — instead, it flashes a temperature number on the display to indicate a problem. When you see the number 90 flashing, the boiler has attempted to light but failed to detect a stable flame, so it has locked out as a precaution. In plain terms: the boiler tried to ignite, couldn't confirm that gas had actually caught and burned, and shut itself down before anything unsafe could happen. For context, 80 flashing on the same range points to an overheat or open-circuit fault, and 70 flashing relates to the air pressure switch — so 90 is specifically and exclusively a flame or gas failure code.
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
- No gas supply reaching the boiler Common
If there is no gas — or insufficient gas pressure — arriving at the boiler, it simply cannot ignite. This could be due to a closed isolation valve, an empty prepayment meter, or a wider supply interruption in your street. Checking whether a gas hob or cooker lights normally is a quick way to rule this out.
- Faulty or dirty ignition electrodes and leads Common
The ignition electrodes generate the spark that lights the burner, and the leads carry the electrical signal to them. Over time, electrodes can become coated with combustion deposits or crack, and leads can develop breaks or poor connections. Either problem can prevent a reliable spark and cause a flame-failure lockout.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas into the burner. If it fails to open correctly — due to electrical failure, a worn actuator, or blockage — gas will not reach the burner in sufficient quantity to sustain combustion. The boiler will attempt to ignite, find no flame, and lock out.
- Blocked or deteriorated burner Sometimes
Carbon deposits can build up on the burner over time, restricting gas flow through the jets and leading to an inconsistent or absent flame. If the burner is heavily fouled, cleaning may restore it; if it is physically damaged, replacement is the safer option.
- Flue or combustion air issue Sometimes
If the flue is partially blocked or a flue seal is missing or damaged, the boiler may not receive adequate fresh air for combustion, or exhaust gases may recirculate into the combustion chamber. Either condition can prevent stable flame establishment. On certain flue lengths, a restrictor plate must also be correctly fitted.
- Faulty PCB (printed circuit board) Rare
The PCB orchestrates the ignition sequence and interprets the flame-detection signal. If it is sending incorrect signals to the gas valve or ignition system, or failing to read the ionisation current from the flame probe, it can cause a spurious flame-failure fault even when the gas and ignition hardware are serviceable.
How to fix it
- Check that the gas supply to your home is working DIY safe
Turn on a gas hob or cooker to see whether gas is reaching your property. If other gas appliances also fail to work, the issue is with your supply rather than the boiler itself. Check that the boiler's gas isolation valve (usually a quarter-turn lever on the gas pipe feeding the boiler) is fully open — the slot in the valve should be aligned with the pipe, not across it. If you have a prepayment meter, ensure it has sufficient credit.
- Reset the boiler once or twice DIY safe
On the Betacom c, turn the central heating control knob to the reset position (marked with a power symbol), hold it for a few seconds, then return it to your desired temperature setting. Some versions have a dedicated reset button instead — check your boiler's label for the exact method. If the boiler fires up and runs normally, monitor it over the next day or so. If the fault returns promptly, do not keep resetting — repeated lockouts signal an underlying fault that needs professional diagnosis.
- Arrange for a Gas Safe registered engineer to carry out a diagnostic inspection Gas Safe engineer
If the gas supply is confirmed and the boiler will not run after one or two resets, a qualified engineer needs to inspect the ignition components, flame probe, gas valve operation, and flue integrity. They will use calibrated test equipment to measure gas pressure at the valve, check the ignition spark, and measure the ionisation current from the flame-detection circuit — none of which can be safely assessed without proper training and tools.
- Have the ignition electrodes and leads inspected and replaced if worn Gas Safe engineer
The engineer will examine the electrodes for cracking, erosion, or heavy deposit build-up, and test the leads for continuity and correct resistance. Worn or contaminated electrodes and leads are one of the most common causes of this fault and are relatively straightforward for an engineer to replace with manufacturer-approved parts.
- Have the gas valve tested and replaced if faulty Gas Safe engineer
A Gas Safe engineer will measure the gas pressure upstream and downstream of the valve and check its electrical actuation. A valve that is not opening to the correct position, or that is leaking, must be replaced by a registered engineer — this is not a homeowner task under any circumstances.
- Have the burner inspected and cleaned or replaced as needed Gas Safe engineer
The engineer will remove the burner for visual inspection and, if appropriate, clean away carbon deposits. If the burner is cracked or heavily corroded, replacement is the safer and more reliable remedy.
- Have the flue, seals, and restrictor plate checked Gas Safe engineer
The engineer should confirm the flue is unobstructed, that all seals are intact, and — depending on the flue run length — that the correct restrictor plate is fitted. A compromised flue can cause combustion problems and represents a carbon monoxide risk, so this check is important even if another component is identified as the primary cause.
Parts you may need
- Ignition electrodes (pair) · from £35
- Ignition leads · from £25
- Flame probe / ionisation electrode · from £20
- Gas valve · from £120
- Burner assembly · 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 £100–£320, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting the boiler when Temperature 90 is flashing?
Resetting once or twice is fine if you have already confirmed the gas supply is on — it rules out a one-off nuisance lockout. However, repeatedly resetting without addressing the root cause is not advisable. If the boiler locks out again quickly, stop resetting and call a Gas Safe engineer. Persistent ignition failures can indicate a gas valve fault, and forcing the boiler through repeated ignition attempts without a confirmed diagnosis is not safe practice.
Could Temperature 90 flashing mean I have a carbon monoxide risk?
A flame-failure lockout means the boiler shut down before combustion could become established, which is actually the safety system working as intended. The immediate risk from this particular fault is no heat or hot water rather than CO exposure. That said, if the underlying cause turns out to be a flue blockage or a damaged burner, there is a potential CO risk when the boiler is running. This is one reason why you should get the flue checked as part of the repair, and why working CO alarms on every floor are strongly recommended in any home with a gas boiler.
How much does it typically cost to fix Temperature 90 flashing on a Glow Worm Betacom c?
For the most common causes — ignition electrodes, leads, or a flame probe — most homeowners pay somewhere between £100 and £200 including parts and labour. A gas valve replacement tends to add up to £250–£320 once parts and engineer time are factored in. Less common repairs such as a PCB or heat exchanger can cost considerably more (£300–£600+), but these are not the typical outcome for this fault code. Glow Worm also offer a fixed-price repair service directly, which averages around £365 and includes a 12-month parts guarantee and 90-day labour guarantee — worth comparing against independent quotes.
What is the difference between Temperature 90, 80, and 70 flashing on a Glow Worm Betacom c?
These three flashing codes each point to a different part of the system. Temperature 90 flashing means the boiler has detected a flame or gas failure — it tried to ignite and could not confirm a flame was established. Temperature 80 flashing indicates an overheat condition or an open circuit on the overheat thermostat circuit, suggesting the boiler got too hot or that the safety thermostat is faulty. Temperature 70 flashing points to a fault with the air pressure switch (APS), which monitors whether the fan is creating sufficient draught for safe combustion. If you are unsure which code your boiler is showing, note whether the number displayed is 70, 80, or 90 before calling an engineer — it will help them prepare the right parts.