BoilerFaultCodesIdeal F 0U Fault Code
Also written as F0U — same fault.
The Ideal F 0U fault code means: your boiler is overheating because the water is not flowing through the system quickly enough to be cooled down.. It appears on 3 Ideal models in our database.
Technical description: Flow/Return Differential greater than 50°C
What causes the Ideal F 0U fault?
This fault occurs when the water temperature leaving the boiler is significantly higher than the water returning, usually because something is blocking the flow. The most common reasons are a failing pump that cannot push the water around your radiators or a buildup of sludge and limescale inside the pipework. When the water sits still or moves too slowly, the heat builds up too fast for the boiler to handle safely.
This issue is most common during the first cold spell of autumn, as pump rotors often seize up after being left idle throughout the summer months.
DIY fix possible
Ensure all radiator valves (TRVs) are turned to the highest settingCheck that the boiler pressure is between 1.0 and 1.5 bar on the gaugeSwitch the boiler off and back on at the fused spur to reset the system
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Ideal models that show the F 0U fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal F 0U 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