BoilerFaultCodesIdeal L5 Fault Code
The Ideal L5 fault code means: your boiler has stopped working because a sensor that monitors the temperature of the water coming back into the unit has detected a problem or failed.. It appears on 11 Ideal models in our database.
Technical description: Return Thermistor Fault
What causes the Ideal L5 fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water returning from your radiators. The most common reason is that the sensor itself has developed an electrical fault or has become corroded over time, though it can also be triggered by loose or damaged internal wiring. Because the boiler can no longer accurately track water temperature, it shuts down as a safety precaution to prevent overheating.
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 L5: 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 L5 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atlantic Combi Repair: £120-220
- Concorde Repair: £0-120
- GB24 / GB30 Repair: £120-220
- i Combi 2 Repair: £90-180
- Instinct2 24 Repair: £90-160
- Logic Code Combi ESP1 Repair: £100-£190
- Logic Combi C24 C30 C35 Repair: £90-180
- Logic Combi ESP1 Repair: £90-160
- Logic Max Combi Repair: £100-£190
- Logic+ Heat H Repair: £90-160
- Logic+ System Repair: £100-180
Source
Ideal L5 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 7 July 2026