ATAG 40 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 40 fault code mean?
ATAG fault code 40 points to a problem with the boiler's return thermistor — the small temperature sensor that monitors the temperature of water flowing back into the boiler after it has circulated around your central heating system. The PCB relies on a continuous stream of accurate readings from this sensor to control burner operation and keep the temperature differential between the flow and return circuits within safe limits. When the sensor sends a reading outside its expected range, or stops communicating altogether, the boiler raises code 40 and may shut down to protect itself. In most cases the fault is a degraded or failed thermistor, though wiring faults and blockages in the return pipework are also known triggers.
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
- Failed or degraded return thermistor Common
Over time the NTC thermistor element deteriorates and its resistance drifts outside the range the PCB expects. This is the most frequent reason code 40 appears, particularly on boilers more than five years old. The sensor may read an implausibly high or low temperature, or produce no signal at all.
- Loose or corroded wiring connections Common
The thermistor connects to the PCB via a plug-and-socket connector. Vibration, age, or dampness inside the boiler casing can cause the pins to loosen or corrode, producing an intermittent or absent signal. Code 40 that clears after a reset but keeps returning is a classic sign of a wiring issue rather than a fully failed sensor.
- Blockage or restricted flow in the return circuit Sometimes
Heavy sludge build-up or a partially closed isolation valve on the return pipe can restrict water movement to the point where the sensor reads abnormal temperatures. A power flush or valve adjustment may resolve the code without any sensor work at all.
- PCB fault misreading the sensor signal Rare
If a new thermistor and verified wiring still produce code 40, the PCB's sensor input circuit may itself be damaged. This is a less common but possible root cause, especially if other sensor-related codes have appeared on the same boiler.
How to fix it
- Check your gas supply is working DIY safe
Before anything else, confirm other gas appliances in your home — such as a hob or gas fire — are working normally. If there is no gas supply at all, contact your gas supplier rather than resetting the boiler.
- Reset the boiler DIY safe
Locate the reset button on your ATAG boiler's control panel (refer to your user manual if unsure). Press and hold it as directed, then allow the boiler to complete its start-up sequence. If code 40 reappears within a short time, do not reset more than two or three times in total — repeated resets without a fix can mask a developing fault.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler. ATAG recommends a cold system pressure of around 1 to 1.5 bar. If it has dropped below 1 bar, top it up via the filling loop following the instructions in your user manual. Low pressure is not a direct cause of code 40 but is worth correcting before calling an engineer.
- Inspect visible pipework and isolation valves DIY safe
Check that all accessible isolation valves on the heating circuit — particularly on the return pipe — are fully open. A valve inadvertently left partially closed can restrict flow and trigger temperature sensor errors.
- Engineer: test and replace the return thermistor Gas Safe engineer
A Gas Safe registered engineer will disconnect the thermistor and test its resistance with a multimeter against the manufacturer's specification chart. If out of range, they will fit a new NTC sensor, applying a small amount of thermal paste to ensure accurate contact with the pipe surface.
- Engineer: inspect and reseat wiring harness connections Gas Safe engineer
If the thermistor tests within spec, the engineer will check the connector pins, wiring harness, and the corresponding input on the PCB for corrosion, damage, or poor contact. Connections will be cleaned or replaced as required.
- Engineer: carry out a system flush if sludge is suspected Gas Safe engineer
If restricted flow is thought to be causing abnormal return temperatures, the engineer may recommend a power flush or chemical flush to clear sludge from the return circuit, followed by a dose of corrosion inhibitor.
- Engineer: assess the PCB if sensor and wiring are confirmed good Gas Safe engineer
Should testing confirm the thermistor and wiring are both in order, the fault likely lies with the PCB's sensor input circuit. A Gas Safe engineer can advise whether repair or board replacement is the appropriate course of action.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault persists after your basic checks and a couple of resets, book a qualified Gas Safe engineer to diagnose and resolve the fault. If your ATAG boiler is still within its warranty period (standard cover is 10 years, extendable to 14 years), contact ATAG Heating UK on 0800 680 0100 before paying for any repairs, as the work may be covered.
Parts you may need
- ATAG return thermistor (NTC sensor) · from £35
- Thermistor wiring harness / connector · from £25
- Thermal paste (for sensor fitting) · from £5
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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix ATAG fault code 40 myself?
The basic checks — confirming gas supply, resetting the boiler, checking system pressure, and making sure isolation valves are fully open — are all safe for a homeowner to carry out. Beyond that, testing or replacing the return thermistor and inspecting PCB wiring requires a Gas Safe registered engineer. Working inside the boiler casing without the right qualifications is illegal for gas appliances in the UK and could invalidate your warranty.
How much does it cost to fix ATAG fault code 40 in the UK?
For a straightforward return thermistor replacement, most homeowners pay between £120 and £280 all-in, covering the call-out fee, labour, and the part itself. The sensor typically costs £15–£50, with the rest being engineer time. Prices in London and the South East tend to run 15–25% higher than the national average. If the fault turns out to be the PCB rather than the sensor, costs will be higher — that scenario is worth discussing with your engineer before authorising the work.
Why does ATAG fault code 40 keep coming back after a reset?
An intermittent fault that clears on reset but reappears is a strong indicator of a loose or corroded connector on the thermistor wiring rather than a completely failed sensor. Temperature changes inside the boiler cause metal components to expand and contract, which can make a borderline connection drop in and out. An engineer can clean and reseat the connector or replace the harness, which usually resolves the pattern permanently.
Is my ATAG boiler still under warranty?
ATAG boilers come with a standard 10-year manufacturer's warranty, which can be extended to up to 14 years under certain conditions. If your boiler is still within this period, contact ATAG Heating UK directly on 0800 680 0100 before booking an independent engineer, as the repair — including parts and labour — may be covered at no cost to you. Make sure any previous servicing has been carried out by a Gas Safe registered engineer, as warranty terms typically require this.