ATAG 101 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 101 fault code mean?
Fault code 101 on an ATAG boiler signals that the unit has detected — or suspects it has detected — an over-temperature condition. Specifically, the boiler's control system monitors the flow sensor (T1) and return sensor (T2), and if either reads above 100 °C for around three seconds, the boiler locks out and displays this code. Because uncontrolled overheating carries serious safety risks, the boiler shuts itself down as a precaution until the underlying cause is identified and resolved. In some cases a faulty sensor gives a false high reading, triggering the lockout even when actual water temperatures are normal. A secondary — less authoritative — interpretation from some trade sources associates code 101 with an ignition sequence problem, but ATAG's own documentation is clear that the primary definition relates to the T1/T2 over-temperature condition.
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
- Faulty flow or return temperature sensor (thermistor) Common
The T1 flow sensor or T2 return sensor can degrade over time, giving inaccurate resistance readings. When the control board receives a signal suggesting water has exceeded 100 °C — even if it hasn't — it triggers an immediate safety lockout. A failed thermistor is one of the most frequent explanations for this code appearing repeatedly after a reset.
- Poor water circulation Common
If water isn't moving through the system properly, heat builds up in the heat exchanger and can genuinely push temperatures beyond the safety threshold. Causes include a worn or seized circulation pump, airlocks in the pipework, or partially closed isolation valves. Without adequate flow, the boiler overheats even if every sensor is reading correctly.
- Limescale or sludge blocking the heat exchanger Common
Deposits of limescale (common in hard-water areas) or magnetite sludge can create hot spots within the heat exchanger where water stops flowing freely. Those localised hot spots push temperatures up rapidly, triggering the over-temperature lockout. This is often accompanied by a kettling noise before the fault appears.
- Gas supply or ignition problem Sometimes
Some trade sources link code 101 to an ignition sequence failure — for example, insufficient gas pressure or a dirty ignition electrode preventing the burner from lighting correctly. While this is a secondary interpretation and not the primary manufacturer definition, it is worth bearing in mind if the boiler fails to fire rather than simply locking out hot.
- PCB fault Rare
In cases where all sensors test within specification and the mechanical system is functioning correctly, an intermittent fault on the printed circuit board can cause it to misinterpret sensor data and issue a false over-temperature lockout. PCB faults tend to show up when all other causes have been ruled out.
How to fix it
- Reset the boiler (up to two or three times only) DIY safe
Locate the reset button on the ATAG control panel — it is typically labelled with a flame or reset symbol. Press and hold it briefly, then allow the boiler to go through its start-up sequence. If the fault clears and the boiler runs normally, monitor it over the next hour or so. If code 101 returns, do not keep resetting it; repeated lockouts mean the underlying cause needs professional attention.
- Check your gas supply is active DIY safe
Try lighting a gas hob or another gas appliance in the property. If those work normally, gas is reaching your home and the supply is unlikely to be the issue. If no gas appliances work, contact your gas supplier rather than an engineer — this is a supply-side problem and no amount of boiler work will fix it.
- Check boiler pressure DIY safe
Look at the pressure gauge on the boiler (or on the app if your model supports it). ATAG boilers operate correctly when cold pressure sits between 1.0 and 1.5 bar. If the reading is below 1.0 bar, top up using the filling loop — consult your user manual for the exact procedure for your model. Low pressure can contribute to poor circulation and indirectly trigger overheating faults.
- Check for a frozen condensate pipe (winter months) DIY safe
If the fault has appeared during cold weather, the condensate drain pipe — usually a white plastic pipe running outside the property — may be frozen, causing the boiler to lock out. Carefully pour warm (not boiling) water along the pipe to thaw it, then reset the boiler. Insulating the pipe afterwards helps prevent a recurrence.
- Have a Gas Safe engineer test the T1 and T2 sensors Gas Safe engineer
An engineer will use a multimeter to measure the resistance of both temperature sensors and compare the readings against the manufacturer's temperature-resistance curve. A sensor reading outside specification will be replaced. This is the most common repair for fault code 101 and should be the first internal inspection the engineer carries out.
- Have the circulation pump inspected and tested Gas Safe engineer
The engineer will check that the pump is running at the correct speed and producing adequate flow. A pump that is noisy, running slowly, or seized will be replaced. They will also check for airlocks and confirm isolation valves are fully open throughout the system.
- Arrange a powerflush if sludge or limescale is suspected Gas Safe engineer
If the engineer finds evidence of magnetite sludge or limescale — particularly in hard-water areas or older systems — a powerflush will be recommended to clean the circuit thoroughly. A magnetic system filter is usually fitted at the same time to keep the system clean going forward.
- Have the PCB inspected if all other checks are clear Gas Safe engineer
If sensors and pump test fine and the system is clean, the engineer may suspect the PCB is misreading sensor data. PCB replacement is a last-resort repair given the cost involved, so a competent engineer will rule out everything else first before recommending it.
- Call a Gas Safe registered engineer Gas Safe engineer
Because fault code 101 involves a genuine overheating risk, any internal work on the boiler must be carried out by a Gas Safe registered engineer. If your boiler is within its 10-year ATAG warranty period, contact ATAG directly on 0800 680 0100 before booking an independent engineer — the repair may be covered at no cost to you.
Parts you may need
- Flow temperature sensor (T1 thermistor) · from £35
- Return temperature sensor (T2 thermistor) · from £35
- Circulation pump · from £120
- Printed circuit board (PCB) · from £280
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
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 £200–£400, depending on the underlying cause.
Frequently asked questions
Is ATAG fault code 101 dangerous — should I keep using my boiler?
You should not attempt to run the boiler while it is displaying fault code 101. The code exists because the boiler's safety system has detected a potential overheating condition, and continued operation without resolving the cause could cause damage to the heat exchanger or, in a worst case, present a safety risk. Leave the boiler off and contact a Gas Safe engineer promptly. If you are cold and waiting for an engineer, use an electric heater as a temporary measure.
Why does my ATAG boiler keep showing fault 101 after I reset it?
If the fault returns quickly after resetting, it means the root cause is still present. Common culprits include a degraded thermistor giving a false high reading, poor water circulation due to a failing pump or sludge build-up, or a genuine heat exchanger hot spot. Repeated resetting without fixing the underlying problem can mask a serious issue, so arrange a professional inspection rather than continuing to reset the boiler.
My ATAG boiler is still under warranty — will the repair be free?
ATAG offers a 10-year manufacturer's warranty on their boilers, provided the unit has been serviced annually by a Gas Safe registered engineer with records to show for it. If your boiler meets those conditions and is within the warranty period, contact ATAG on 0800 680 0100 before booking anyone independently. If the fault is covered, you could save several hundred pounds on parts and labour. Book an independent engineer first and you may forfeit any warranty contribution.
Could fault code 101 simply be a bad sensor rather than real overheating?
Yes — this is actually one of the most common scenarios. A thermistor degrades naturally over time, and a sensor that reports 105 °C when the actual water temperature is 65 °C will still trigger the lockout. An engineer can confirm this quickly with a multimeter: if the sensor resistance is out of specification for the measured flow temperature, a straightforward sensor replacement (typically £200–£350 including labour) resolves the fault. Real overheating due to pump failure or sludge is also common, but a false reading from a worn sensor is frequently the explanation for code 101 on otherwise well-maintained boilers.