BoilerFaultCodesIdeal 4.9 Fault Code
Also written as 49 — same fault.
The Ideal 4.9 fault code means: your boiler has detected an issue with a temperature sensor that monitors the water returning from your radiators, often resulting in the system shutting down for safety.. It appears on 9 Ideal models in our database.
Technical description: Return Thermistor Fault
What causes the Ideal 4.9 fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has developed an internal electrical failure or the wiring connecting it to the main computer has become loose or damaged. Because the boiler can no longer safely track how much heat is being removed from the system, it shuts down 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 4.9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 4.9 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 Logic Code Combi ESP1 manual (PDF)
- Ideal Logic Max Combi manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026