ATAG 50 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 50 fault code mean?
Fault code 50 on ATAG iC-Range combi boilers indicates a problem with the domestic hot water (DHW) temperature sensor, known internally as the T3 sensor. The boiler has detected an open circuit, short circuit, or a temperature reading that falls outside the acceptable range on this sensor. Because the boiler can no longer reliably measure the temperature of the hot water it is producing, it locks out as a safety precaution and prompts you to press the reset button on the control panel. This fault is specific to the iC-Range combi boilers; the iS and iR system and regular boiler ranges use a slightly different fault code set.
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 DHW temperature sensor Common
The DHW sensor (T3 probe) is in direct contact with the hot water circuit and is subject to heat cycling every time hot water is drawn. Over time the sensor element can degrade, drift out of calibration, or fail outright, triggering the fault code even when everything else is working correctly.
- Open or short circuit in the sensor wiring Common
The wiring between the DHW sensor and the PCB can develop a break (open circuit) or an unintended electrical path (short circuit) due to heat damage, vibration, or age. Either condition causes the PCB to read an impossible signal and lock the boiler out with code 50.
- Loose or corroded connector plug Sometimes
The connector that joins the sensor wiring harness to the PCB or to the sensor itself can work loose over time, or the pins can corrode — especially in a humid boiler cupboard. A poor connection mimics the same electrical symptoms as a full open circuit.
- Scale or sludge contamination around the sensor Sometimes
In hard-water areas, limescale can build up around the sensor probe, effectively insulating it from the water it is meant to measure. Sludge from an unclean system can have a similar effect, causing the sensor to report temperatures that are outside the expected range.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button on the boiler control panel as described in your user manual. Wait for the boiler to complete its restart sequence. If the fault clears and does not return within a short period of normal use, it may have been a transient glitch. However, if code 50 reappears — or returns repeatedly — do not keep resetting indefinitely, as this will not address the underlying problem and may mask a worsening fault.
- Check that your boiler is still under warranty DIY safe
ATAG offers a 10-year warranty as standard on their iC-Range boilers, with the heat exchanger covered for life, and this can be extended to 14 years. Before spending anything on repairs, locate your warranty documents or contact ATAG Heating directly. If the boiler is within the warranty period and has been serviced annually as required, the sensor replacement may be covered at no cost to you.
- Contact a Gas Safe registered engineer to inspect the DHW sensor and wiring Gas Safe engineer
A qualified engineer will use a multimeter to test the resistance of the T3 DHW sensor against the manufacturer's expected values at a given temperature. They will also inspect the wiring harness for any signs of heat damage, chafing, or breaks, and check the connector plugs for corrosion or loose pins. This diagnostic step is essential before committing to replacing any parts.
- Clean or replace the DHW temperature sensor Gas Safe engineer
If the sensor is contaminated with scale or sludge, the engineer may be able to clean it and refit it. Where the sensor has failed electrically or is physically damaged, it will need to be replaced. The part itself is relatively inexpensive, and replacement is usually a straightforward job for an experienced engineer.
- Repair or replace the sensor wiring harness Gas Safe engineer
If the fault lies in the wiring rather than the sensor itself, the engineer will repair or replace the affected section of the harness and ensure all connector plugs are seated correctly. After any wiring work the boiler should be tested through a full DHW draw to confirm the fault is resolved.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If code 50 continues to appear after the sensor and wiring have been checked and addressed, the engineer may need to investigate the PCB, which in rare cases can misread a perfectly healthy sensor due to an internal fault. PCB diagnosis and replacement must be carried out by a Gas Safe registered professional.
Parts you may need
- DHW temperature sensor (T3 probe) — ATAG iC-Range · from £35
- Sensor wiring harness · from £25
- Connector plug / terminals · from £10
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 50 myself?
The only thing you should do yourself is attempt a single reset using the button on the control panel. Everything beyond that — inspecting the sensor, testing wiring, or replacing components — must be carried out by a Gas Safe registered engineer. Working inside the boiler without the correct qualifications is illegal for gas appliances and could invalidate your warranty.
Will my ATAG warranty cover a fault code 50 repair?
It may well do. ATAG provides a 10-year warranty as standard on iC-Range boilers, extendable to 14 years, provided the boiler has been serviced annually by a qualified engineer. Contact ATAG Heating or your registered installer before paying for any repair, and have your warranty documentation to hand.
How much does it cost to fix fault code 50 on an ATAG boiler?
For most homeowners this comes down to a call-out, a short labour session, and the cost of a new DHW sensor — typically somewhere between £120 and £280 all in. If the fault turns out to involve a wiring harness repair as well, costs sit towards the higher end of that range. In rare cases where the PCB is implicated, bills can be significantly higher, but that outcome is uncommon for this particular fault code.
Why does fault code 50 only appear on iC-Range ATAG boilers?
ATAG uses different control systems and fault code sets across their product ranges. The code 50 DHW sensor error is specific to the iC-Range combi boilers. If you have an iS or iR system or regular boiler, the same underlying problem may be reported under a different code number, so it is always worth checking your model's own documentation or contacting ATAG directly if you are unsure.