BoilerFaultCodesWorcester Bosch 661 Fault Code
The Worcester Bosch 661 fault code means: your boiler is not receiving the correct electrical voltage needed to run, likely due to an issue with your home's power supply or a failing internal circuit board.. It appears on 1 Worcester Bosch model in our database.
Technical description: Unsatisfactory condition - power supply
What causes the Worcester Bosch 661 fault?
This fault occurs when the boiler's internal electronic brain detects that the incoming electricity is at the wrong voltage or frequency to operate safely. The most common reason is a failing Printed Circuit Board (PCB) that can no longer process power correctly, though it can also be triggered by external issues like a faulty main power supply or loose electrical connections.
This fault is slightly more common during winter or stormy weather, when local power surges or grid fluctuations are more likely to interfere with sensitive boiler electronics.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the 661 fault code
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Source
Worcester Bosch 661 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026