Overview
Error code 50 on ATAG iC / iS combi boilers means the boiler has detected a fault with the domestic hot water (DHW) temperature sensor (often called T3 or the hot water probe). The sensor reports temperatures that are out of the expected range or it is not responding, so the boiler either disables hot water or locks out as a safety measure. This fault is specific to hot water sensing and typically does not mean there is an immediate gas leak or fire risk, but it does mean the boiler cannot reliably control DHW temperature. Common causes are a failed sensor, a damaged or corroded wiring harness/connector, a poor connection, water ingress around the sensor, or (less commonly) a PCB/communication fault. Severity is moderate: you will usually lose hot water or get intermittent hot water, and the boiler may go into a lockout if the fault persists. Some causes can be checked by a careful homeowner (visual checks and a reset), but diagnosing and replacing sensors or any internal electrical/PCB work should be done by a qualified Gas Safe engineer. Do not attempt gas or sealed-component repairs yourself.
Possible Cause: Domestic hot water (DHW) sensor error
Troubleshooting Steps
Safety first
1) Do not work on gas parts. If you are not competent with mains electricity or boiler internals, stop and call a Gas Safe registered engineer.
2) Always switch the boiler off at the external fused spur before touching wiring or opening the casing. Allow the boiler to cool and isolate the mains supply if you plan any internal inspection.
3) Be aware of stored hot water and hot surfaces; allow time to cool before touching pipework.
Initial homeowner checks (safe, non-invasive)
1) Note exactly when the code appears (during hot water demand, when turning a tap, or random). Reproduce if needed to confirm.
2) Try a single reset of the boiler using the normal reset button or turning the boiler off and on at the fused spur. If the code clears and does not return, it may have been a transient reading. If it returns immediately or after a short time, further checks are needed.
3) Check the boiler display for any other codes or the spanner/service symbol. If the boiler has locked out, the display will usually indicate that.
4) Visually inspect around the boiler for signs of water leakage or corrosion which could affect the sensor wiring. Also check that the hot water taps and any thermostatic mixing valves are functioning correctly.
If you are willing and competent to perform basic visual/electrical checks (only if comfortable and with power isolated)
1) Isolate the mains power at the fused spur. Confirm the boiler is off. If unsure, do not proceed.
2) With the cover still on, look for accessible wiring and connectors to the DHW sensor (consult the boiler manual for the sensor location if available). Check for loose connectors, damaged insulation, corrosion or moisture at the connector. Tighten or dry connections if safe to do so.
3) If you can safely access the sensor wiring and you have a multimeter and the manual with expected thermistor values, you can measure resistance across the sensor with the wiring harness unplugged. Note: thermistors change resistance with temperature (NTC decreases as temperature rises). If the sensor reads open circuit (infinite) or a value far from the manual’s expected range, the sensor is faulty and needs replacement.
4) If the sensor resistance looks plausible, re-check the wiring harness and connectors for intermittent faults (wiggle test while monitoring if you have the skills). Intermittent wiring faults can cause the error.
5) Clean around the probe area if there is visible debris or corrosion—thermistors sometimes need cleaning rather than replacement. Do not use aggressive tools or solvents that could damage wiring or components.
If the fault remains after these checks
1) Do not attempt to replace internal components beyond the sensor unless you are a qualified engineer. Repeatedly trying to force fixes can create unsafe conditions or invalidate warranties.
2) Contact a Gas Safe registered heating engineer. Tell them you have ATAG iC / iS combi boiler fault code 50 (DHW sensor / T3 hot water sensor error), what checks you have already carried out (reset, visual inspection, any multimeter readings), and whether the fault is permanent or intermittent.
3) The engineer will verify sensor readings, replace the DHW sensor if required, check the sensor wiring and connectors, and if necessary test the PCB and bus communications. They will also perform safety and operational checks after repair and clear any lockouts.
Important notes
1) Replacing a sensor is often straightforward for a qualified technician but involves working inside the boiler and potentially draining parts of the system—this is not recommended for unqualified DIYers.
2) If the fault is caused by an internal PCB or intermittent wiring to the PCB, a professional is required to diagnose and replace components safely.
3) If multiple or recurring sensor faults occur, mention this to the engineer as it may indicate an underlying moisture, pump or system fault affecting probes.
When to call a professional immediately
1) If you see signs of water leaks inside the boiler casing, corrosion on electrical connectors, or if the boiler locks out and you cannot restore safe operation by a single reset.
2) If you cannot safely access or test the sensor, or if any electrical measurements are outside expected values. A Gas Safe engineer should carry out replacement and full safety checks.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag IC & IS Combi Boiler.