Overview
Error code 101 on an ATAG boiler is reported by ATAG documentation as an overheat error. This means the boiler has detected temperatures above a predefined safe limit and has shut down or gone into a lockout to protect the appliance and your heating system. Common causes include poor water circulation (blocked pipework, closed valves, airlock or failed pump), a restricted heat exchanger (scale or debris), faulty temperature sensors or probe wiring, or other faults that stop heat being removed from the boiler. Severity is medium to high: the boiler has shutdown to avoid damage or unsafe conditions, so it is not safe to ignore. In many cases a homeowner can perform basic, non-intrusive checks (power/reset, system pressure, bleeding radiators, visual checks) to try to clear a temporary cause. However, because overheating can be caused by failed components inside the boiler (pump, sensor, heat exchanger, wiring or controls) and because work on gas appliances and internal components must only be done by a Gas Safe qualified engineer, you should call a qualified technician if the problem persists after the basic checks, or immediately if you see leaking, continuous pressure faults, repeated lockouts or smell gas.
Possible Cause: Overheat error
Troubleshooting Steps
Safety first
- If the boiler is displaying an overheat code, do not ignore it. Turn the boiler off using the control panel or the isolate switch and allow it to cool for at least 20–30 minutes before doing anything else. Do not open the boiler casing. Never attempt to work on gas or sealed electrical components unless you are Gas Safe qualified.
- If you smell gas, evacuate the property, ventilate where safe, do not operate electrical switches or the boiler, and call the gas emergency number immediately.
Initial homeowner checks (safe, non-intrusive)
1. Confirm the exact code and consult the boiler manual or front panel legend—some sources list 101 as overheat but other references can differ; record what you see for the engineer.
2. After the boiler has cooled, attempt a single reset using the boiler’s reset button (follow the manual). Wait and observe. If the error returns immediately or repeatedly, stop resetting and proceed to professional help.
3. Check the system water pressure on the boiler gauge. Typical operating pressure is about 1.0–2.0 bar cold. If the pressure is very low, repressurise to the level recommended in the manual. If the pressure is abnormally high (>3 bar) or you see water discharging from the pressure relief valve, turn the boiler off and call an engineer.
4. Bleed radiators to remove trapped air which can cause poor circulation and overheating. After bleeding, re-check and top up system pressure if needed.
5. Check that radiator thermostatic valves and the central heating isolating valves on the boiler are open and set to allow flow. Ensure the room thermostat or timer hasn’t forced the boiler into an unusual mode.
6. Listen/feel for the circulating pump when the boiler is calling for heat (briefly turn the boiler on if cooled and safe to do so). If the pump is not running, or is noisy or stuck, do not attempt to disassemble it—note the symptom and call an engineer.
7. Look for obvious leaks, signs of scale, or blockage of external condensate pipes (in freezing weather condensate blockages can cause other faults). Clear external blockages only if they are accessible and safe to do so.
Specific diagnostic steps and safe fixes you can try
1. Single reset: after cooling, press reset once and let the boiler attempt to restart. If the problem was a transient circulation or sensor glitch the boiler may return to normal. Do not perform repeated resets.
2. Repressurise the system if pressure is low to the correct cold pressure stated in your manual (usually ~1–1.5 bar). Re-test. Low pressure and airlocks are common, simple causes of overheating because flow is reduced.
3. Bleed radiators to remove airlocks and improve flow. Re-pressurise if required and try a reset again.
4. Check external condensate discharge for ice or blockage in cold weather and thaw/clear where safe. A blocked condensate pipe more commonly causes ignition/lockout errors but is worth confirming if present and accessible.
5. If you find the pump is not running but the boiler has power, do not open the boiler; record what you heard/saw and call an engineer.
When to call a professional
- If the 101 error returns after the basic checks and one reset, call a Gas Safe qualified heating engineer. Overheat faults commonly need internal diagnosis: failed flow/return temperature probes, a blocked or scaled heat exchanger, pump failure, faulty PCB or faulty safety thermostat/limit. These parts require specialist tools and safe handling.
- Call an engineer immediately if you notice persistent high pressure, water discharging from the relief valve, repeated lockouts, leaks, or any smell of gas.
What to tell the engineer
- The exact fault code (101) and any additional codes or symbols. What you observed when the fault occurred (boiler on/off, radiators hot or cold, noises). Any steps you have already taken (reset, repressurised, bled radiators). Recent conditions (freezing weather, system work, recent servicing).
Do not attempt
- Do not open the boiler casing or attempt to adjust internal wiring, gas valves, or replace sensors unless you are qualified. Do not repeatedly reset the boiler to “clear” a fault. Do not work on the gas supply yourself.
Summary
- 101 is an overheat/temperature-safety fault on many ATAG lists. Try safe, non-intrusive checks first (cool and reset once, check pressure, bleed radiators, ensure valves and pump are allowing flow). If the fault persists, or if you see leaks, pressure relief activity, or other worrying symptoms, switch the boiler off and contact a Gas Safe registered heating engineer for diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.