BoilerFaultCodesIdeal L4 or F4 Fault Code
Also written as L4ORF4 — same fault.
The Ideal L4 or F4 fault code means: your boiler's temperature-sensing component is malfunctioning, which prevents it from safely controlling the heat, so it has turned itself off to prevent overheating.. It appears on 13 Ideal models in our database.
Technical description: FLOW THERMISTOR FAULT
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A cover plan would handle faults like L4 or F4: no call-out fees, no surprise bills.
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Ideal models that show the L4 or F4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal L4 or F4 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 31 August 2026