S.04
FAULT CODE
Vaillant
Blocking: Return Sensor Too Hot
Medium Severity
Engineer Required
The return temperature is too high — the boiler is in a blocking state and will restart automatically when temperature drops.
Step-by-Step Fix Guide
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Step 1
Allow the boiler to cool naturally
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Step 2 Engineer only
If this code appears frequently, a Gas Safe engineer should check circulation and pump settings
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.