BoilerFaultCodesIdeal L04 Fault Code
The Ideal L04 fault code means: your boiler has detected a problem with the internal sensor that monitors the temperature of the water leaving the unit, causing it to stop working for safety.. It appears on 3 Ideal models in our database.
Technical description: Flow Thermistor Fault
What causes the Ideal L04 fault?
This fault occurs when the boiler's control system receives an incorrect or missing electrical signal from the sensor that monitors hot water leaving the unit. It is usually caused by the internal sensor failing due to age, or a loose connection in the wiring that prevents the boiler from safely regulating its temperature.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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Gas Safe Register →Ideal models that show the L04 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal L04 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026