BoilerFaultCodesIdeal Flow Thermistor Fault Fault Code
Also written as FLOWTHERMISTORFAULT — same fault.
The Ideal Flow Thermistor Fault fault code means: your boiler has detected that a temperature sensor is not communicating correctly, which has caused the system to stop working to prevent overheating.. It appears on 5 Ideal models in our database.
Technical description: Insecure wiring to Flow Thermistor (upper LHS of heat exchanger), incorrect resistance values (9.7-10.3 kΩ at 25°C), or insecure PCB connections.
What causes the Ideal Flow Thermistor Fault fault?
This fault occurs when the boiler's internal computer loses its ability to accurately monitor the temperature of the water leaving the heat exchanger. The most common reason is that the thermistor sensor has physically degraded or developed an electrical failure, but it can also be caused by a loose connection or a fault within the main control board. Because the boiler can no longer ensure the water isn't getting dangerously hot, it shuts down as a safety precaution.
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 Flow Thermistor Fault fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal Flow Thermistor Fault 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