BoilerFaultCodesIdeal L05 Fault Code
The Ideal L05 fault code means: your boiler has lost its ability to accurately measure the temperature of the water coming back into the system, which is preventing it from heating up correctly.. It appears on 1 Ideal model in our database.
Technical description: Return Thermistor Fault
What causes the Ideal L05 fault?
This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler fails or provides an illogical reading. The most common reason is that the internal component has suffered an electrical failure or the wiring connecting it to the main control board has become loose or damaged.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Ideal models that show the L05 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal L05 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 7 July 2026