BoilerFaultCodesIdeal F04 L04 Fault Code
Also written as F04L04 — same fault.
The Ideal F04 L04 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, usually causing the system to shut down as a safety precaution.. It appears on 3 Ideal models in our database.
Technical description: Flow Thermistor
What causes the Ideal F04 L04 fault?
This fault occurs when the internal sensor responsible for measuring the water temperature as it leaves the boiler fails or provides an unrealistic reading. The most common reason is that the sensor has physically degraded over time or a loose electrical connection has developed, preventing the control board from safely monitoring the heat levels.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Ideal models that show the F04 L04 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal F04 L04 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