BoilerFaultCodesIdeal L4 Fault Code
The Ideal L4 fault code means: your boiler has stopped working because it cannot correctly detect how much water is flowing through the system or what the temperature is.. It appears on 9 Ideal models in our database.
Technical description: Control / no flow thermistor fault
What causes the Ideal L4 fault?
This fault occurs when the boiler's internal sensors provide conflicting or impossible temperature readings, making it unsafe for the unit to continue heating. The most common cause is a faulty flow thermistor that has failed electrically, or a significant lack of water circulation preventing the heat from moving away from the heat exchanger. In some cases, a communication error on the main circuit board can lead the boiler to incorrectly report this flow issue.
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 L4 fault code
Click a model for the full fault code list and model-specific repair notes.
- Independent C24 Repair: £120-250
- Logic Code Combi ESP1 Repair: £100-£180
- Logic Combi C24 C30 C35 Repair: £90-160
- Logic Combi ESP1 Repair: £90-160
- Logic Max & Logic Plus Repair: £100-£220
- Logic Max Combi Repair: £100-£180
- Logic Max Plus Repair: £100-£220
- Logic Max System Repair: £100-£180
- Logic+ System Repair: £120-220
Source
Ideal L4 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