43 Boiler Fault Code: What It Means on Every Brand

The 43 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 43 means	Severity	DIY?
JSJohnson & Starley	Code 43 on a Johnson & Starley warm-air unit points to a short circuit on the return air temperature sensor — the probe measuring the temperature of air being drawn back into the heater… Full guide →	Engineer may be needed	No
NNibe	Fault code 43 on a Nibe heat pump indicates that the three-phase electrical supply is connected in the wrong order. Full guide →	Lockout — engineer likely	No

43 brand by brand

Johnson & Starley 43

Code 43 on a Johnson & Starley warm-air unit points to a short circuit on the return air temperature sensor — the probe measuring the temperature of air being drawn back into the heater from the house. The controller uses this reading to manage heat output and fan behaviour; a sensor shorted (typically to earth, or across its own conductors) returns an impossible value, so the unit flags code 43. The documented action is to inspect the wiring connections and check the sensor's resistance. Its neighbouring code 44 covers the return sensor's other failure modes (open or short on the central-heating return on combined models), making 43/44 a diagnostic pair around return-side temperature sensing.

Causes, fixes & costs for Johnson & Starley 43 → · All Johnson & Starley codes

Nibe 43

Fault code 43 on a Nibe heat pump indicates that the three-phase electrical supply is connected in the wrong order. The unit's internal monitoring detects that the phase rotation is incorrect and immediately blocks the compressor from running as a protective measure. While the fault is active, the heat pump cannot produce heating or domestic hot water.

Causes, fixes & costs for Nibe 43 → · All Nibe codes

Frequently asked questions

Is the 43 fault code the same on every boiler?

No. 43 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 43 fault myself?

Not usually. On every brand listed, 43 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 43 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.