L4 Boiler Fault Code: What It Means on Every Brand

The L4 fault code appears on 2 brands — Grant and Ideal — 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 L4 means	Severity	DIY?
GGrant	L4 on a Grant Aerona³ heat pump reports a mix water temperature sensor error: the thermistor measuring temperature at a mixing point — used on installations with a mixed (blended) circuit,… Full guide →	Engineer may be needed	No
IIdeal	The L4 fault code on Ideal boilers indicates that the boiler's control system has detected a problem with the flow thermistor — the temperature sensor that monitors how hot the water is as… Full guide →	Lockout — engineer likely	No

L4 brand by brand

Grant L4

L4 on a Grant Aerona³ heat pump reports a mix water temperature sensor error: the thermistor measuring temperature at a mixing point — used on installations with a mixed (blended) circuit, most commonly underfloor heating running cooler than the radiator circuit — has failed or reads implausibly. The controller blends flow water down to the mixed circuit's target using that reading; without it, it cannot guarantee the underfloor loops are protected from full-temperature flow, so the affected circuit stops. Grant's documented diagnosis is a resistance check with a tester, and the remedy the standard one: a faulty sensor is replaced. This code auto-resets once the reading returns to normal.

Causes, fixes & costs for Grant L4 → · All Grant codes

Ideal L4

The L4 fault code on Ideal boilers indicates that the boiler's control system has detected a problem with the flow thermistor — the temperature sensor that monitors how hot the water is as it leaves the heat exchanger. Because the boiler cannot safely manage water temperature without a reliable reading from this sensor, it shuts down as a precaution. You may also see this displayed as F4 on certain Ideal models such as the Logic Max; both codes point to the exact same underlying issue, just presented differently depending on the model variant. The boiler will remain locked out until the fault is properly resolved and cleared.

Causes, fixes & costs for Ideal L4 → · All Ideal codes

Frequently asked questions

Is the L4 fault code the same on every boiler?

No. L4 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 L4 fault myself?

Not usually. On every brand listed, L4 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 L4 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.