ATAG 110 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 110 fault code mean?
Fault code 110 on an ATAG boiler indicates that the appliance has detected an unsafe condition — either the water temperature has climbed beyond its safe upper limit (an overheat lockout), or system water pressure has risen to a dangerously high level. In either scenario the boiler shuts itself down as a protective measure. The overheating interpretation is the more widely referenced meaning, and ATAG's own fault-finding guidance notes that a very rapid rise in flow temperature — sometimes as much as 7–15 °C within a single second — can point to a sudden loss of water flow across the heat exchanger. Left unaddressed, repeated overheat events can cause secondary damage to components such as the pump, thermistor, or even the PCB, so prompt investigation is important.
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
- Restricted water circulation / sludge and scale build-up Common
Magnetite sludge, limescale, and debris accumulating inside the heat exchanger or pipework reduce the flow of water through the boiler. With less water moving over the heat exchanger, temperatures spike rapidly, triggering the safety cut-out. Noisy pipework and slow-to-warm radiators are tell-tale signs alongside this fault code.
- Circulation pump failure or reduced output Common
If the pump is running slowly, intermittently, or has seized, water cannot circulate at the rate the boiler expects. The heat exchanger overheats quickly, and the boiler locks out. A Gas Safe engineer can test pump head pressure and replace the unit if needed.
- Faulty flow temperature sensor (thermistor) Common
The flow thermistor continuously monitors the temperature of water leaving the heat exchanger. If it develops a fault — reporting temperatures that are too high, too low, or erratically — the boiler's control board may interpret this as an overheat event and shut down, even when the actual water temperature is within safe limits.
- High system water pressure Sometimes
Some ATAG models and third-party sources also associate code 110 with system pressure that has risen above the safe maximum (typically above 3 bar). Overpressure stresses internal components and can trigger a safety lockout. Check the pressure gauge on the boiler front — a reading above roughly 2.5 bar when cold warrants investigation.
- Blocked or dirty air filter / restricted flue Sometimes
Restricted airflow caused by a clogged air filter or partially obstructed flue can cause combustion and heat-transfer problems that contribute to overheating. Filters should be inspected and cleaned as part of an annual service.
- Water leak reducing system volume Sometimes
A leak anywhere in the system — even a slow drip at a radiator valve or a pinhole in pipework — progressively lowers the volume of water available for heat transfer, causing the remaining water to overheat. Look for damp patches, staining around pipework, or a pressure gauge that keeps dropping.
How to fix it
- Do not repeatedly reset the boiler DIY safe
A single reset attempt is reasonable — press the reset button on the ATAG control panel and observe whether the boiler lights and runs normally. If the fault code 110 returns within a short time, stop resetting. Repeated resets on an overheating boiler can cause further damage to internal components and should be avoided.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. For most ATAG models, the normal cold-system pressure is between 1.0 and 1.5 bar. If the needle is sitting above roughly 2.5–3 bar, system pressure may be contributing to the fault. Do not attempt to raise pressure if it is already high. If pressure appears normal or low, move to the next step.
- Bleed radiators if pressure is high DIY safe
If pressure reads above around 2.5 bar, bleeding one or two radiators can release trapped air or excess pressure. Use a radiator bleed key, hold a cloth beneath the bleed valve, and open it slowly until water (not air) trickles out, then close it. Re-check the gauge. If pressure remains high after bleeding, do not manipulate the filling loop or expansion vessel — call an engineer.
- Check for visible leaks around the system DIY safe
Visually inspect accessible pipework, radiator valves, and any visible boiler connections for signs of dripping water, staining, or dampness. If you find a leak, turn the boiler off and call an engineer promptly. Low system volume from a leak is a common driver of overheating.
- Check that the gas supply to the property is on DIY safe
Confirm other gas appliances (hob, gas fire) are working normally. If there is no gas supply, contact your gas supplier rather than an engineer. This step is unlikely to be the direct cause of fault 110 but rules out a concurrent issue that could confuse diagnosis.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
If the fault persists after the basic checks above, a Gas Safe engineer needs to inspect the boiler. They will measure pump performance, test the flow and return thermistors with a multimeter, check the heat exchanger for scale or blockage, inspect the expansion vessel pre-charge pressure, and review the system for sludge contamination. Do not attempt to open the boiler casing or interfere with the pump, thermistor, gas valve, or PCB yourself.
- Power flush or chemical flush if sludge is confirmed Gas Safe engineer
If the engineer identifies sludge or scale as the root cause, a power flush — using specialist equipment to force clean water and descaling chemicals through the entire system — will be required. This must be carried out by a competent heating engineer. Following the flush, fitting a magnetic system filter (such as a Magnaclean) helps prevent future build-up.
- Replace faulty components as diagnosed Gas Safe engineer
Depending on findings, the engineer may need to replace the circulation pump, the flow thermistor, the expansion vessel, or — if overheating has caused collateral damage — the PCB. All component replacement on an ATAG boiler must be carried out by a Gas Safe registered engineer to comply with UK regulations and to preserve the manufacturer warranty.
Parts you may need
- Flow temperature thermistor (NTC sensor) · from £25
- Circulation pump · from £120
- Expansion vessel · from £55
- Magnetic system filter (e.g. Magnaclean Pro2) · from £65
- PCB (control board) · from £280
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 £150–£450, depending on the underlying cause.
Frequently asked questions
Is ATAG fault code 110 dangerous — should I keep using the boiler?
You should not keep resetting and running the boiler if fault code 110 keeps returning. The code signals that your boiler has reached an unsafe temperature or pressure and shut itself down to protect you and the system. Repeatedly overriding a safety lockout risks damaging the heat exchanger, pump, or PCB, and in extreme cases could create a more serious hazard. Turn the boiler off and arrange for a Gas Safe registered engineer to inspect it promptly.
How much does it typically cost to fix ATAG fault code 110 in the UK?
For the most common causes — a pump replacement, thermistor swap, or a descale and system clean — most homeowners pay somewhere between £150 and £450 all-in, including labour. A straightforward thermistor replacement at the lower end, or a pump replacement with a call-out fee at the higher end, covers the majority of cases. If the engineer finds that heavy sludge contamination requires a full power flush, costs can reach £350–£850 depending on the number of radiators and the degree of contamination — this is worth budgeting for separately. In the rare situation where overheating has damaged the PCB, an additional £300–£500 may be needed. If your ATAG boiler is still within its 10-year warranty period, contact ATAG on 0800 680 0100 before arranging any third-party repair, as covered work could be carried out at no cost.
Can I fix ATAG fault code 110 myself?
Homeowners can safely carry out a small number of checks: attempting a single boiler reset, reading the pressure gauge, bleeding radiators if pressure is high, and looking for obvious leaks. Beyond that, the internal causes of this fault — a failing pump, faulty thermistor, scaled heat exchanger, or PCB damage — all require a Gas Safe registered engineer. Working on gas boiler components without Gas Safe registration is illegal in the UK and will void your ATAG warranty.
Why does my ATAG boiler keep showing code 110 after I reset it?
If fault code 110 returns shortly after each reset, the underlying problem has not been resolved. Likely culprits include a circulation pump that is struggling to maintain adequate flow, a thermistor giving inaccurate temperature readings, or significant sludge and scale build-up that keeps causing the heat exchanger to overheat. Persistently resetting a boiler in this condition risks causing additional damage. Arrange an engineer visit so the root cause can be properly diagnosed and fixed rather than temporarily masked by repeated resets.