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 12 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.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like L4 or F4: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Ideal models that show the L4 or F4 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atlantic Combi Repair: £120-220
- CLASSIC2 Repair: £150-250
- ESPRIT ECO2 Repair: £150-250
- EXCLUSIVE2 Repair: £150-250
- i-mini C24 / C30 Repair: £90-180
- INDEPENDENT + COMBI Repair: £150-250
- Independent C24 Repair: £150-250
- INSTINCT2 Repair: £150-250
- Instinct2 24 Repair: £150-250
- Logic Code Combi ESP1 Repair: £150-250
- Logic Combi C24 C30 C35 Repair: £120-220
- Logic Combi ESP1 Repair: £120-200
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