Quick answer
E120 on a Baxi boiler is a flue-overheat safety shutdown: the temperature of the gases leaving through the flue has risen above the maximum permitted level (around 140°C), so the boiler has locked itself out to prevent damage and protect your home. Common triggers are a partially blocked or restricted flue, a fouled heat exchanger, or abnormal combustion.
You may try one reset, but if the code returns, stop — this is an engineer-only fault. Do not keep resetting and do not open the boiler. Book a Gas Safe registered engineer to find and fix the underlying cause before the boiler is used again.
What does E120 mean on a Baxi boiler?
E120 is a flue-overheat fault. Your Baxi boiler constantly monitors the temperature of the combustion gases leaving through the flue. When that temperature exceeds the maximum safe limit — typically around 140°C — the boiler triggers a safety shutdown and displays E120.
It is doing exactly what it is designed to do: shutting down before excessive heat can damage components or create a safety risk.
The E120 code sits in the same family as Baxi's other temperature- and overheat-related faults (such as E110). Because the exact wording and threshold can vary slightly between Baxi model ranges and platforms, the engineer will read the fault against your specific boiler's manual.
What is consistent across the range is the principle: the flue gases are running hotter than permitted, and that needs investigating before the boiler runs again.
Do not repeatedly reset a boiler showing E120. The lockout is a safety feature responding to abnormally hot flue gases. Forcing it back on without fixing the cause risks damaging the heat exchanger and other parts — and could mask a genuine combustion or flue problem.
One reset to clear a one-off glitch is reasonable; after that, leave it for a Gas Safe engineer.
Common causes of E120
An over-temperature flue reading usually points to one of the following:
- A blocked or restricted flue — debris, nests, or a partial obstruction stopping hot gases venting freely.
- A fouled or failing heat exchanger — sludge, scale, or blockage causing heat to build up instead of transferring into the water.
- Poor circulation — a jammed pump, trapped air, or heavy sludge meaning water isn't carrying heat away from the heat exchanger fast enough.
- Abnormal combustion — an incorrectly set or faulty gas valve, dirty burner, or fan fault changing how the boiler burns.
- A faulty flue-gas temperature sensor — occasionally the sensor itself reports a false high reading.
All of these involve the gas, flue, or sealed combustion side of the boiler — which is why E120 is firmly engineer territory.
What you can safely check
E120 is an engineer-only fault, so there is no DIY repair here — but there are a few universal, safe checks any homeowner can make before booking an engineer:
- Try one reset. Use the reset button as described in your manual, or see our guide on how to reset your boiler. If E120 clears and stays away, it may have been a one-off. If it returns, stop and call an engineer.
- Check the gas is on. Make sure the gas supply isn't off and other gas appliances are working.
- Glance at the pressure gauge. Confirm the visible pressure is roughly 1–1.5 bar when cold. See what your boiler pressure should be and, if it's low, boiler pressure too low.
- In cold weather, check the condensate pipe. A frozen external condensate pipe can be thawed safely with warm (not boiling) water — though this is more commonly linked to other fault codes than E120.
That is the limit of safe homeowner action. Do not remove the boiler casing, inspect the flue, or touch the burner, gas valve, fan, or heat exchanger.
When to call a Gas Safe registered engineer
If E120 returns after a single reset, book a Gas Safe Register engineer. By law, only a Gas Safe registered engineer may work on the gas supply, burner, gas valve, flue, sealed combustion circuit, PCB, or pressure-relief valve. It is illegal and dangerous for anyone unregistered to attempt this work.
A competent engineer investigating E120 will typically:
- Inspect the flue for blockages, restrictions, or incorrect installation.
- Check combustion with a flue-gas analyser and verify gas pressures.
- Examine the heat exchanger for sludge, scale, or blockage, and check circulation and the pump.
- Test the flue-gas temperature sensor and wiring.
- Confirm the fan and burner are operating correctly before returning the boiler to service.
If you ever smell gas or suspect a leak, leave the property and call the National Gas Emergency line on 0800 111 999. This is also worth understanding alongside a general boiler lockout, which is the wider category E120 belongs to.
Typical Baxi E120 repair cost
Costs depend heavily on the underlying cause and your location. These are indicative 2026 UK ranges, including labour, to give you a sense of what to expect.
| Work | Indicative 2026 cost |
|---|---|
| Diagnostic / call-out | £70 – £120 |
| Clearing a blocked flue / minor obstruction | £100 – £250 |
| Flue-gas temperature sensor replacement | £120 – £250 |
| Power flush to clear sludge / restore circulation | £400 – £800 |
| Gas valve replacement (see faulty gas valve) | £250 – £450 |
| Heat exchanger replacement | £400 – £800+ |
For wider context on what different jobs cost, see our guide to boiler repair costs. A simple flue clearance is at the cheaper end; a heat exchanger replacement can approach the cost of a new boiler on an older unit.
Related Baxi codes
If you're comparing codes, these sibling faults are worth a look:
- Baxi E110 — boiler maximum temperature exceeded / overheat.
- Baxi E119 — low water pressure in the heating circuit.
- Baxi E133 — gas supply / ignition fault.
You can also browse the full Baxi fault codes hub for the complete list.
Will boiler cover pay for an E120 repair?
The following is general information, not financial advice or a recommendation to buy any particular policy. Always check the specific terms of any plan before deciding.
It depends on your policy. Many boiler cover plans will pay for an engineer to diagnose and repair a fault like E120 — including parts and labour — provided the boiler was in good working order when the policy started and has been serviced as required.
However, providers commonly exclude pre-existing faults, sludge-related damage where there's no system filter, and boilers above a certain age, so it's worth reading the small print.
To understand what's typically included, see what boiler cover includes, and weigh up whether a policy makes sense for you with is boiler cover worth it.
As a rough guide, a one-off fault on a newer boiler may cost less than a year of cover, while an older boiler prone to repeat issues may make cover more appealing — but only you can judge what's right for your circumstances.
Is a Baxi E120 fault dangerous?
The fault itself is the boiler protecting you — it has shut down because the flue gases got too hot. That's a safety feature working correctly.
The underlying cause, however, can be serious (a blocked flue or abnormal combustion), so it must be diagnosed by a Gas Safe registered engineer before the boiler is used again. If you ever smell gas, call 0800 111 999.
Can I fix E120 myself?
No. Every likely cause involves the flue, heat exchanger, gas valve, or combustion — all of which only a Gas Safe registered engineer may legally work on. The only safe homeowner actions are a single reset, checking the gas is on, glancing at the pressure gauge, and thawing an external condensate pipe in cold weather.
Will E120 clear on a reset?
It might clear briefly if it was a one-off glitch, and trying one reset is reasonable. But if the flue gases are genuinely running too hot, the code will come straight back. Don't keep resetting — repeatedly forcing the boiler on can damage the heat exchanger and hides a real fault.
How much does it cost to repair a Baxi E120?
Expect roughly £70–£120 for diagnosis, with the total depending on the cause: a flue clearance might be £100–£250, a power flush £400–£800, and a heat exchanger replacement £400–£800 or more. Your location and boiler age affect the figure.
Why does my Baxi keep showing E120?
A code that keeps returning points to a persistent cause — most often a restricted flue, a sludged or failing heat exchanger, poor circulation, or a combustion problem. None of these clear on their own. Book a Gas Safe engineer to find and fix the root cause rather than resetting repeatedly.
Tired of one-off repair bills?
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Compare boiler coverThis article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.