BoilerFaultCodesYour boiler's main internal computer has encountered a communication error and cannot control the heating process correctly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Control mode - Internal error
What causes this fault?
This fault occurs when the boiler's central brain suffers a hardware failure or a critical data mismatch within its internal circuits. The most common reason is a breakdown of the communication between the main circuit board and the specialized coding plug that stores the boiler's specific operating instructions.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like b5 — no call-out fees, no surprises.
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Viessmann Vitodens 333 is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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