FU
FAULT CODE
Ideal
Temperature Difference Too High — Check Isolation Valves
Medium Severity
DIY Check Possible
The temperature difference between the flow and return is greater than 50°C. The boiler is blocking to prevent damage. Almost always caused by closed isolation valves or a seized pump.
Step-by-Step Fix Guide
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Step 1
Check that ALL radiator thermostatic valves are open (including the lockshield valves)
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Step 2
Check that any isolation valves in the boiler pipework are fully open
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Step 3
Bleed radiators
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Step 4
Reset the boiler
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Step 5
If FU returns, call a Gas Safe engineer to check the pump
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.