BoilerFaultCodesIdeal F05 L05 Fault Code
Also written as F05L05 — same fault.
The Ideal F05 L05 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing it from heating up correctly.. It appears on 3 Ideal models in our database.
Technical description: Return Thermistor
What causes the Ideal F05 L05 fault?
This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that monitors the water returning from your radiators. The most common reason is that the sensor has physically failed due to age or electrical wear, though it can also be caused by a loose connection or damaged wiring inside the unit.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Ideal models that show the F05 L05 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal F05 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 27 April 2026