L5 Boiler Fault Code: What It Means on Every Brand

The L5 fault code appears on 3 brands — Grant, Ideal and Keston — 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 L5 means	Severity	DIY?
GGrant	L5 on a Grant Aerona³ heat pump reports a humidity sensor error: a humidity measurement the controller has been configured to use — typically for cooling-mode dew-point protection, where… Full guide →	Engineer may be needed	No
IIdeal	The L5 code is a safety lockout on Ideal boilers that covers two closely linked problems. Full guide →	Lockout — engineer likely	Some checks
KKeston	The L5 code on a Keston boiler signals that the control board has detected a problem with the return thermistor — the sensor responsible for measuring the temperature of water coming back… Full guide →	Lockout — engineer likely	No

L5 brand by brand

Grant L5

L5 on a Grant Aerona³ heat pump reports a humidity sensor error: a humidity measurement the controller has been configured to use — typically for cooling-mode dew-point protection, where the system must not chill surfaces below the temperature at which condensation forms — has failed or is reading implausibly. Grant's documented diagnosis is a resistance check with a tester, and the remedy is replacement of the faulty sensor. On heating-only installations without a humidity sensor fitted, an L5 can instead point to a configuration mismatch — a control setting expecting a sensor that is not there — which is a settings correction rather than a parts job.

Causes, fixes & costs for Grant L5 → · All Grant codes

Ideal L5

The L5 code is a safety lockout on Ideal boilers that covers two closely linked problems. Most commonly, it means the boiler has detected a fault with the return pipe thermistor — the sensor that monitors the temperature of water flowing back into the boiler. On some models it can also trigger when the boiler has restarted itself five or more times within a 15-minute window, which is Ideal's built-in protection against repeated ignition or circulation failures. Either way, the boiler shuts down completely and will not restart on its own until the root cause is resolved and the fault is manually cleared. The official Ideal guidance sums it up simply: there is a problem with the return side of the system.

Causes, fixes & costs for Ideal L5 → · All Ideal codes

Keston L5

The L5 code on a Keston boiler signals that the control board has detected a problem with the return thermistor — the sensor responsible for measuring the temperature of water coming back into the boiler after it has circulated around the radiators and underfloor circuits. Without a reliable reading from this sensor, the boiler cannot safely regulate burner output, so it shuts down into a lockout state. On Keston boilers, L-prefix codes denote a hard lockout: the boiler will not restart on its own and requires a manual reset once the underlying fault is addressed. You may also see this fault referenced as F5 on some displays — F5 is the non-lockout version of the same underlying issue, whereas L5 means the boiler has already shut itself down. Keston boilers are Ideal-derived machines with a twin-flue condensing design, which means not every engineer is familiar with them — it is worth checking that any engineer you call has hands-on Keston experience.

Causes, fixes & costs for Keston L5 → · All Keston codes

Frequently asked questions

Is the L5 fault code the same on every boiler?

No. L5 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 L5 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 L5 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.