ATAG 61 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 61 fault code mean?
Fault code 61 on an ATAG boiler signals that the boiler's internal monitoring has detected a problem with water flow through the heat exchanger circuit. In plain terms, the boiler performed a 'flow check' and it failed — meaning either the flow sensor itself has reported an unexpected reading, or the actual movement of water around the system is insufficient. The boiler locks out as a safety precaution to prevent the heat exchanger from overheating or running dry. In some cases this is a genuine hydraulic problem (pump, valves, blockage); in others it is an electrical fault with the sensor or its wiring rather than a true flow problem.
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 or failed flow sensor Common
The flow sensor (sometimes called the flow temperature sensor or flow NTC) monitors the temperature and behaviour of water leaving the heat exchanger. If the sensor has drifted out of calibration, developed an internal fault, or failed outright, the boiler will log a code 61 even when water flow itself is perfectly normal. This is the most frequently reported root cause.
- Circulation pump failure or reduced output Common
If the pump is not moving water at the expected rate — due to worn bearings, an airlock inside the pump head, or a seized impeller — the flow check will fail. The boiler may also show this code during initial start-up if the pump is slow to prime.
- Closed or partially closed isolating valves Sometimes
Service valves or zone valves that are inadvertently shut or only partially open restrict flow enough to trigger a failed flow check. This can happen after recent plumbing or boiler service work.
- Blockage or heavy sludge in the system Sometimes
A build-up of magnetite sludge or limescale within the pipework, heat exchanger, or filter can restrict circulation to the point where the boiler cannot confirm adequate flow. Systems without a magnetic filter are particularly vulnerable.
- Loose or corroded wiring to the flow sensor Sometimes
Vibration or moisture can cause the electrical connector at the flow sensor to work loose or corrode over time. A poor connection produces an intermittent or out-of-range signal that the boiler interprets as a sensor fault.
- Internal electronic fault (PCB) Rare
In a smaller number of cases, the PCB misinterprets or fails to read the sensor signal correctly due to a board-level fault. This is typically identified after other causes have been ruled out.
How to fix it
- Check that all service valves and zone valves are fully open DIY safe
Walk around your heating system and confirm that any isolating valves on the flow and return pipes — and any zone or motorised valves — are fully open. A valve that was closed during recent maintenance and not reopened is an easy, cost-free fix. Handles in line with the pipe mean open; handles at 90° to the pipe mean closed.
- Reset the boiler DIY safe
Switch the boiler off at the mains or use the reset button, leave it powered down for at least 10 minutes, then restart. This allows the system to re-initialise and gives the pump time to clear any minor airlock. Only attempt a reset two or three times — repeated resets without resolving the underlying fault can mask more serious issues.
- Check and top up system pressure if low DIY safe
Look at the pressure gauge on the boiler. For most ATAG models the operating pressure should sit between 1.0 and 1.5 bar when cold. If it is below 0.8 bar, use the filling loop to top it up to around 1.2 bar. Low pressure alone rarely triggers code 61, but very low pressure can starve the pump and contribute to a failed flow check.
- Inspect the magnetic filter (if fitted) DIY safe
If your system has a magnetic filter (such as a Magnaclean or similar), check whether it is heavily loaded with sludge. A blocked filter severely restricts flow. You can visually inspect the filter body; if it is clearly full, an engineer should clean it as part of the diagnostic visit. Do not attempt to dismantle pressurised components yourself.
- Engineer to inspect the flow sensor and its wiring Gas Safe engineer
A Gas Safe registered engineer will use a multimeter to check the resistance of the flow sensor against the manufacturer's specification and inspect the wiring harness and connector for signs of corrosion, damage, or a loose pin. A faulty sensor is replaced as a unit; a corroded connector can sometimes be cleaned and re-seated.
- Engineer to check and service the circulation pump Gas Safe engineer
The engineer will confirm the pump is running, test its speed settings, and check for airlocks or a seized impeller. A pump that has worn bearings or is running at reduced output will be replaced. On ATAG boilers the pump is typically an integral component, so this work must be carried out by a qualified engineer familiar with the model.
- Engineer to power-flush or chemically clean the system if sludge is found Gas Safe engineer
If significant magnetite sludge or limescale is found to be restricting flow, a power-flush or targeted chemical clean may be recommended. This is a multi-hour job and, on heavily contaminated systems, is often the single biggest factor in preventing repeat faults. A magnetic filter should be fitted afterwards if one is not already present.
- Engineer to test or replace the PCB if all other causes are excluded Gas Safe engineer
If the sensor, wiring, pump, and hydraulic system all check out correctly, the fault may lie with the PCB's sensor-reading circuit. PCB diagnosis and replacement must be carried out by a Gas Safe registered engineer. Note that on an ATAG boiler still within its 10-year warranty (extendable to 14 years with annual servicing), PCB replacement may be covered — contact ATAG before authorising any major parts.
- Call a Gas Safe registered engineer if the fault persists after your initial checks Gas Safe engineer
If the boiler does not clear after resetting and checking valves and pressure, do not continue resetting it repeatedly. Book a Gas Safe registered engineer who has experience with ATAG boilers to carry out a full diagnostic. Bring your boiler's installation date and service history, as this will determine whether warranty cover applies.
Parts you may need
- ATAG flow temperature sensor (NTC) · from £28
- Wiring harness / sensor connector · from £18
- Circulation pump (ATAG compatible) · from £95
- Magnetic system filter (e.g. Magnaclean Pro2) · from £55
- PCB (model-specific) · from £260
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
Is ATAG fault code 61 dangerous — should I stop using the boiler?
The boiler will typically lock itself out when code 61 appears, so it will not continue operating unsafely. There is no immediate gas or carbon monoxide risk associated with a flow sensor fault. However, you should not repeatedly override the lockout by resetting the boiler without identifying the cause. Leave the boiler off and book an engineer if the fault returns after one or two resets.
My ATAG boiler is still under warranty — will fault code 61 be covered?
ATAG offers a standard 10-year parts and labour warranty, extendable to 14 years, provided the boiler has been serviced annually by a Gas Safe registered engineer. If your boiler is within warranty and has been serviced as required, the cost of diagnosing and fixing a flow sensor or pump fault may be covered in full. Contact ATAG's customer support line before booking an independent engineer, as using an unauthorised repairer for warranty work could affect your cover.
How much does it cost to replace a flow sensor on an ATAG boiler?
The sensor itself typically costs around £20–£35 for the part. Add a Gas Safe engineer's call-out and labour (usually one hour of work) and the total bill is commonly in the range of £120–£200. If the fault turns out to be the circulation pump, expect to pay £200–£380 including parts and labour. A PCB replacement is significantly more expensive — often £350–£500 or more — but this is an uncommon outcome for code 61 and is worth mentioning to your engineer only if simpler causes have been ruled out.
Can I fix ATAG fault code 61 myself?
There are a handful of checks a homeowner can safely carry out: confirming all valves are open, topping up system pressure via the filling loop, and resetting the boiler once or twice. Beyond that, the diagnosis involves working on electrical components (the sensor, wiring, PCB) and pressurised hydraulic parts (the pump, pipework) — all of which require a Gas Safe registered engineer. Attempting these yourself would also void your ATAG warranty.