Glow-worm F70 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F70 fault code mean?
The F70 code appears when the boiler's control panel (user interface) and its main PCB cannot recognise each other due to a software or firmware version mismatch. Essentially, the two electronic components are speaking different languages — one may have been replaced with a unit running a different firmware revision, or a non-approved board has been installed. To protect itself from unpredictable behaviour, the boiler enters a lockout state and displays F70 until the incompatibility is resolved. Because this is an electronic control fault rather than a gas or heating-circuit issue, it almost always requires a Gas Safe registered engineer with access to Glow-Worm diagnostic tools.
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
- Wrong PCB version fitted after a previous repair Common
The single most common trigger for F70. If a PCB was recently replaced — perhaps during a different repair visit — there is a strong chance the replacement board carries a different firmware revision to the one the control panel expects. Even boards that look physically identical may carry incompatible software versions, and Glow-Worm's control system is strict about version matching.
- Non-approved or generic replacement PCB installed Sometimes
Some engineers source cheaper, non-Glow-Worm-approved PCBs to keep costs down. Generic boards often lack the correct firmware entirely, causing the boiler to reject the pairing and lock out with F70 immediately after installation.
- Loose or damaged wiring between the control panel and PCB Sometimes
A poor connection on the ribbon cable or harness that links the user interface to the main board can disrupt communication enough to mimic a version mismatch. Although the underlying electronics may be compatible, intermittent signal loss causes the boiler to log F70 as a safety measure.
- Firmware not updated after PCB swap Rare
Certain Glow-Worm models support firmware updates via manufacturer tools. If a newer PCB was fitted but the firmware was never flashed to the version the control panel requires — or vice versa — the boards remain out of step and F70 is triggered.
How to fix it
- Attempt a single boiler reset DIY safe
On most Glow-Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold for a few seconds, then release and set to your desired temperature. Wait 60 seconds to see if the boiler fires. Because F70 is an electronic lockout, a reset alone is very unlikely to clear it unless the fault was a one-off communication glitch — but it costs nothing to try once before calling an engineer.
- Check for any recent PCB or control-panel work DIY safe
Think back over the last few months. Has an engineer replaced the PCB, control panel, or any electronic component? If so, note the engineer's details and the parts used — this information will help the next engineer identify whether a mismatched board was fitted, potentially saving diagnostic time and money.
- Do not attempt repeated resets DIY safe
If the first reset does not clear F70, do not keep resetting. The fault is rooted in an electronic incompatibility that resetting cannot fix, and repeated lockout cycles put unnecessary strain on the ignition components.
- Engineer to verify PCB and control-panel firmware compatibility Gas Safe engineer
A Gas Safe registered engineer will cross-reference the part numbers and firmware revisions of the installed PCB and control panel against Glow-Worm's compatibility matrix. This step pinpoints whether the mismatch is in hardware, software, or both.
- Engineer to inspect and reseat the wiring harness between the two boards Gas Safe engineer
Before ordering parts, a competent engineer will check the ribbon cable and connector pins linking the user interface to the PCB. A loose or corroded connection can sometimes produce F70, and reseating it is a low-cost first fix worth eliminating.
- Engineer to install the correct PCB or reflash firmware Gas Safe engineer
If the wrong board was fitted, the engineer will source a Glow-Worm-approved PCB with the correct firmware revision for your specific boiler model and software generation. Where the boiler supports it, a firmware update via Glow-Worm's service tool may resolve the mismatch without a full board swap.
- Call a Gas Safe registered engineer if not already done Gas Safe engineer
F70 is not a fault a homeowner can resolve at home. If you have not already contacted an engineer, do so now. Ask specifically for someone familiar with Glow-Worm controls — they are more likely to carry compatible stock and have access to the manufacturer's diagnostic software.
Parts you may need
- Glow-Worm main PCB (model-specific) · from £220
- Glow-Worm user interface / control panel assembly · from £95
- PCB to control panel wiring harness / ribbon cable · from £25
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 £300–£550, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F70 fault myself?
No — F70 is entirely an electronic control fault involving the PCB and control panel. There are no gas components to adjust, but the repair still requires a Gas Safe registered engineer with access to Glow-Worm's compatibility data and diagnostic tools. The one homeowner-safe step is a single reset attempt, which will rarely clear F70 on its own.
Why has my boiler suddenly shown F70 after a repair?
This is the classic F70 scenario. If an engineer recently replaced the PCB or another electronic component, there is a good chance the replacement part carries a firmware version that is incompatible with your control panel. Contact the engineer who carried out the work and ask them to verify the board version against Glow-Worm's approved compatibility list — they should rectify this at no extra charge if they supplied the wrong part.
How much does it cost to fix a Glow-Worm F70 fault?
Most homeowners pay between £300 and £550 for a PCB swap including parts, labour, and VAT. The PCB itself typically costs £180–£280 for an approved Glow-Worm unit, with the remainder covering the engineer's time. If the fault turns out to be a loose wiring connection rather than a board mismatch, the bill can be considerably lower — sometimes just a call-out and labour fee. In rare cases involving specialist or discontinued models where parts are hard to source, costs can exceed £600, which may make a new boiler worth considering instead.
Is it worth repairing an F70 fault, or should I replace the boiler?
If your boiler is under ten years old and otherwise in good condition, a PCB replacement that clears F70 is usually a worthwhile investment — it can restore several more years of reliable service. However, if the boiler is older than ten years, has had multiple faults recently, or the repair quote exceeds roughly half the cost of a new A-rated boiler, replacement often makes better financial sense in the long run, especially given the energy-efficiency gains from a modern condensing boiler.