40 Boiler Fault Code: What It Means on Every Brand

The 40 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 40 means	Severity	DIY?
AATAG	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… Full guide →	Lockout — engineer likely	No
NNibe	Nibe fault code 40 has two distinct meanings depending on your model. Full guide →	Lockout — engineer likely	No
VViessmann	Fault code 40 on a Viessmann boiler indicates that the burner control unit has triggered a safety lockout because the fan was not at a complete standstill when expected, or because the… Full guide →	Lockout — engineer likely	No

40 brand by brand

ATAG 40

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.

Causes, fixes & costs for ATAG 40 → · All ATAG codes

Nibe 40

Nibe fault code 40 has two distinct meanings depending on your model. On ground-source and older systems, code [40] indicates that the voltage on compressor phase 1 has dropped below 160 V for more than 30 minutes — the compressor is blocked and will not run until the phase supply is restored and the alarm is reset. On air-source models using the EB101 outdoor unit (e.g. the F2040 range), the same number appears as part of alarm 220 displayed as E:40, which is a high-pressure alarm caused by restricted water flow through the heat exchanger. Both variants lock out the compressor to prevent damage and need to be investigated before you attempt a reset.

Causes, fixes & costs for Nibe 40 → · All Nibe codes

Viessmann 40

Fault code 40 on a Viessmann boiler indicates that the burner control unit has triggered a safety lockout because the fan was not at a complete standstill when expected, or because the plug-in connection to the fan is unstable. Viessmann boilers continuously monitor fan speed (RPM) throughout the heating cycle — including before ignition and after shutdown. If the fan is still spinning when it should have stopped, or if the control unit receives an erratic or missing speed signal, it locks the boiler out rather than risk unsafe combustion. This fault can appear on all Viessmann boiler models and always requires investigation by a Gas Safe registered engineer.

Causes, fixes & costs for Viessmann 40 → · All Viessmann codes

Frequently asked questions

Is the 40 fault code the same on every boiler?

No. 40 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 40 fault myself?

Not usually. On every brand listed, 40 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a 40 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.