45 Boiler Fault Code: What It Means on Every Brand

The 45 fault code appears on 2 brands — Johnson & Starley and Nibe — 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 2 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 45 means	Severity	DIY?
JSJohnson & Starley	Fault Code 45 appears on Johnson & Starley WarmCair warm air boilers when the control system detects a short circuit in the flue gas temperature sensor circuit. Full guide →	Lockout — engineer likely	No
NNibe	Nibe alarm code 45 indicates that the single-phase motor protection cut-out switch inside the heat pump has tripped, interrupting power to the compressor. Full guide →	Lockout — engineer likely	No

45 brand by brand

Johnson & Starley 45

Fault Code 45 appears on Johnson & Starley WarmCair warm air boilers when the control system detects a short circuit in the flue gas temperature sensor circuit. In practice, the boiler displays 'E45' and shuts down to prevent unsafe operation. The boiler cannot reliably monitor flue gas temperatures, so it locks out as a precaution. This is closely related to Code 46, which indicates an open circuit on the same sensor — engineers often find the two codes alternate depending on the exact nature of the wiring fault, and the diagnostic approach is similar for both. All work to clear this fault must be carried out by a Gas Safe registered engineer with warm air appliance experience.

Causes, fixes & costs for Johnson & Starley 45 → · All Johnson & Starley codes

Nibe 45

Nibe alarm code 45 indicates that the single-phase motor protection cut-out switch inside the heat pump has tripped, interrupting power to the compressor. This is a safety mechanism designed to prevent damage when abnormal current is detected. Until the underlying cause is identified and the fault is cleared, the unit will not provide heating or hot water.

Causes, fixes & costs for Nibe 45 → · All Nibe codes

Frequently asked questions

Is the 45 fault code the same on every boiler?

No. 45 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 45 fault myself?

Not usually. On every brand listed, 45 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 45 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.