ATAG Fault Code 111: Maximum Temperature Exceeded
ATAG fault code 111 indicates that the boiler has detected a water temperature above its maximum permitted threshold and has locked out as a safety precaution. The boiler will not restart until the underlying cause has been identified and resolved. In some ATAG models or third-party references the same code is also linked to ignition failure, so if a reset does not clear the fault a Gas Safe engineer should confirm the exact trigger on your specific appliance.
General guidance only — not a substitute for professional advice. 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.
What does the ATAG 111 fault code mean?
ATAG fault code 111 indicates that the boiler has detected a water temperature above its maximum permitted threshold and has locked out as a safety precaution. The boiler will not restart until the underlying cause has been identified and resolved. In some ATAG models or third-party references the same code is also linked to ignition failure, so if a reset does not clear the fault a Gas Safe engineer should confirm the exact trigger on your specific appliance.
Common causes
- Restricted water circulation Common
A partially closed isolation valve, a failing circulation pump, or a heavily sludged system can all reduce water flow through the heat exchanger. When water moves too slowly it absorbs heat faster than it can carry it away, pushing the flow temperature past the boiler's safety limit and triggering the lockout.
- Sludge or limescale build-up Common
Years of corrosion deposits and limescale accumulating in the heat exchanger narrow the internal waterways. The restricted flow causes localised hot spots, which are picked up by the temperature sensors and cause the boiler to shut down on overheat.
- Faulty flow thermistor (send probe) Sometimes
The flow thermistor monitors outgoing water temperature and feeds that reading back to the PCB. If the sensor has drifted out of calibration or failed completely it can report a falsely high temperature, causing the boiler to lock out even when actual temperatures are normal.
- System pressure loss or leak Sometimes
A drop in system pressure — caused by a leak at a radiator valve, a joint, or the pressure relief valve — reduces the volume of water in the circuit. Less water means heat concentrates more quickly, raising temperatures beyond safe limits.
- Gas valve or ignition fault (model-dependent) Sometimes
On certain ATAG variants, code 111 is associated with an ignition failure rather than an overheat condition. Issues such as a weakened spark electrode, a corroded ionisation pin, or a gas valve not opening correctly can all prevent a successful flame establishment and generate this code.
- PCB fault Rare
The printed circuit board controls how the boiler responds to sensor readings. A PCB that is misreading signals, suffering from moisture ingress, or failing intermittently can incorrectly trigger an overheat lockout even when all physical components are operating within normal parameters.
How to fix it
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up slowly using the filling loop until you reach approximately 1.2 bar, then close the filling loop valves securely. Do not overfill — pressure above 2.5 bar is itself a fault condition.
- Confirm the gas supply is live DIY safe
Check that other gas appliances in your home (a gas hob, for example) are working normally. If nothing gas-powered will operate, contact your gas supplier rather than your boiler engineer. If the gas supply is fine, move on to the next step.
- Reset the boiler DIY safe
Locate the reset button on your ATAG boiler's control panel — it is usually marked with a flame symbol or labelled 'Reset'. Hold it for a few seconds, then allow the boiler to complete its restart sequence. Only attempt a reset two or three times. If the fault code returns each time, further resets will not help and may mask the underlying problem.
- Check all radiator and isolation valves are fully open DIY safe
Walk around your home and confirm that every radiator thermostatic valve and any manual isolation valves on the pipework are fully open. A partly closed valve elsewhere in the circuit can dramatically restrict flow and cause the boiler to overheat.
- Arrange a Gas Safe engineer inspection Gas Safe engineer
If the fault persists after the checks above, a qualified Gas Safe engineer should attend. They will measure flow rates, inspect the circulation pump, test the flow thermistor for accuracy, check the heat exchanger for sludge or scale, and — where code 111 is ignition-related — examine the spark electrode, ionisation pin, and gas valve wiring. Do not attempt to access gas components, the PCB, or internal sensors yourself.
- Consider a power flush if sludge is identified Gas Safe engineer
If the engineer finds significant sludge or limescale in the system, a power flush will be recommended to clear the blockages and restore proper water flow. Following a flush, fitting a magnetic system filter will help prevent a recurrence. This work must be carried out by a qualified heating engineer.
- Replace faulty components as diagnosed Gas Safe engineer
Depending on the engineer's findings, parts such as the flow thermistor, circulation pump, gas valve, or PCB may need replacing. Only a Gas Safe registered engineer should source and fit these components on an ATAG boiler, particularly to preserve any remaining manufacturer warranty.
Parts you may need
- Flow thermistor (send probe) · from £25
- Circulation pump · from £120
- Pressure relief valve · from £35
- Ignition electrode · from £30
- Gas valve · from £110
- PCB (printed circuit board) · 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–£380, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my ATAG boiler to clear fault code 111?
You can try a reset two or three times to see whether the fault was a one-off event — a brief temperature spike during very cold weather, for instance. However, if the code returns each time the boiler restarts, repeated resets are simply masking a problem that will not fix itself. Continuing to reset without investigation also risks damage to the heat exchanger. Book a Gas Safe engineer if the fault recurs.
How much does it typically cost to fix ATAG fault code 111 in the UK?
Most repairs that cause this fault — a flow thermistor swap, a pump replacement, or a system power flush — fall in the region of £120 to £380 including labour. A gas valve replacement sits towards the top of that range at around £200–£250 all in. If the PCB turns out to be faulty, costs can rise to £400–£500 or more; at that point, and especially if the boiler is out of warranty, it may be worth comparing the repair cost against a new boiler installation.
Will fault code 111 affect my ATAG warranty?
ATAG boilers carry a standard 10-year warranty, extendable to 14 years on some models, but this is conditional on annual servicing by a qualified engineer. If the overheat lockout is caused by a lack of servicing — sludge build-up or a failed component that should have been caught at a service — your warranty claim could be affected. Always keep records of annual services and use ATAG-approved engineers where possible.
Could fault code 111 mean an ignition problem rather than overheating on my ATAG boiler?
Yes, on certain ATAG models the same code can be linked to ignition failure rather than a temperature issue. If your boiler is locking out but the system pressure and flow appear normal, an ignition-related cause — such as a worn spark electrode, a corroded ionisation pin, or a gas valve fault — should be investigated. A Gas Safe engineer can read the boiler's internal diagnostics to confirm which trigger is active on your specific model.