D4
FAULT CODE
Worcester Bosch
System Purge Required / Air in System
Low Severity
DIY Check Possible
Air has been detected in the primary circuit after a period of non-use or maintenance work. The boiler may need purging (air bleeding).
Step-by-Step Fix Guide
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Step 1
Bleed all radiators starting from the ground floor
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Step 2
Check system pressure and repressurise to 1.5 bar if needed
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Step 3
Reset the boiler
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Step 4
If D4 persists, a Gas Safe engineer should carry out a system purge
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.