BoilerFaultCodesIdeal F 04 OR L 04 Fault Code
Also written as F04ORL04 — same fault.
The Ideal F 04 OR L 04 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely control the heat.. It appears on 3 Ideal models in our database.
Technical description: FLOW THERMISTOR FAULT
What causes the Ideal F 04 OR L 04 fault?
This fault occurs when the boiler's internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the water being sent to your taps or radiators. The most common reason is that the sensor itself has developed a physical defect or the electrical wiring connecting it to the main control board has become damaged or loose. Without this data, the boiler shuts down as a safety precaution to prevents it from potentially overheating.
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 F 04 OR L 04 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal F 04 OR L 04 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