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 6 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.
- Ideal Atlantic Combi manual (PDF)
- Ideal Instinct2 24 manual (PDF)
- Ideal Logic Code Combi ESP1 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026