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 19 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.
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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
- Classic Repair: £120-250
- EXCLUSIVE2 Repair: £90-180
- GB24 / GB30 Repair: £120-220
- i Combi 2 Repair: £90-180
- Independent Repair: £120-250
- INDEPENDENT + COMBI Repair: £90-180
- Independent C24 Repair: £120-250
- INDEPENDENT SYSTEM Repair: £90-180
- Independent System S15 Repair: £90-180
- INSTINCT2 Repair: £90-180
- Logic Code Combi ESP1 Repair: £100-£190
- Logic Combi C24 C30 C35 Repair: £90-180
- Logic Combi ESP1 Repair: £90-160
- LOGIC HEAT Repair: £90-180
- Logic Max Heat Repair: £90-180
- Logic Max System Repair: £100-£190
- LOGIC SYSTEM Repair: £90-180
- Logic+ Heat H Repair: £90-160
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.
- Ideal GB24 / GB30 manual (PDF)
- Ideal Independent System S15 manual (PDF)
- Ideal Logic+ Heat H manual →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026