50 Boiler Fault Code: What It Means on Every Brand

The 50 fault code appears on 3 brands — ATAG, Nibe and Viessmann — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 3 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What 50 means	Severity	DIY?
AATAG	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. Full guide →	Lockout — engineer likely	No
NNibe	Fault code 50 on a Nibe heat pump indicates that the high-pressure switch has been activated more than once in quick succession. Full guide →	Lockout — engineer likely	Some checks
VViessmann	Fault code 50 on Viessmann boilers — most commonly seen on the Vitodens 200-W and 222-F — signals that domestic hot water (DHW) production has been suspended because the boiler has… Full guide →	Engineer may be needed	No

50 brand by brand

ATAG 50

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.

Causes, fixes & costs for ATAG 50 → · All ATAG codes

Nibe 50

Fault code 50 on a Nibe heat pump indicates that the high-pressure switch has been activated more than once in quick succession. When this happens, the unit locks out the compressor and requires a manual reset before it will run again. In plain terms, the refrigerant pressure on the high side of the system has climbed beyond a safe threshold — and because it happened repeatedly rather than as a one-off blip, the heat pump refuses to restart automatically as a protective measure. In the vast majority of cases, the root cause is not the refrigerant circuit itself but poor water flow through the heating-side heat exchanger, which prevents the heat pump from shedding heat efficiently and causes pressure to rise.

Causes, fixes & costs for Nibe 50 → · All Nibe codes

Viessmann 50

Fault code 50 on Viessmann boilers — most commonly seen on the Vitodens 200-W and 222-F — signals that domestic hot water (DHW) production has been suspended because the boiler has detected a short circuit on the DHW tank temperature sensor (NTC). In plain terms, the control unit cannot get a reliable temperature reading from the hot water circuit, so it shuts down DHW as a precaution. Your central heating should continue to work normally, but taps and showers will run cold. Codes 51, 58, and 59 sit in the same DHW sensor/fault family: code 51 typically indicates an open circuit (broken signal) rather than a short, while 58 and 59 relate to further DHW sensor or circuit deviations — if you see any of these alongside or instead of 50, the same diagnostic approach applies.

Causes, fixes & costs for Viessmann 50 → · All Viessmann codes

Frequently asked questions

Is the 50 fault code the same on every boiler?

No. 50 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a 50 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a 50 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For heat pumps, an MCS or F-Gas qualified heat pump engineer. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.