BoilerFaultCodesIdeal E9 Fault Code
The Ideal E9 fault code means: your heat pump has stopped working because a sensor that monitors the temperature of the cooling liquid returning to the main unit has developed a fault.. It appears on 1 Ideal model in our database.
Technical description: Compressor Refrigerant Return Thermistor (Th) Error (E9)
What causes the Ideal E9 fault?
This fault occurs when the system's brain loses track of the temperature of the refrigerant liquid as it flows back into the compressor. The most common reason is a failed electrical sensor that has either burnt out or is sending back scrambled readings that don't make sense. It can also be caused by a loose connection or a damaged wire between that sensor and the main control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Ideal models that show the E9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal E9 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