BoilerFaultCodesYour boiler's internal computer is having trouble processing electrical signals correctly, which is preventing it from running its safety and efficiency tests.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Emissions test mode - Internal error analog converter
What causes this fault?
This fault occurs when the boiler's main brain fails to translate physical electrical signals into data it can understand. It is usually caused by an internal hardware failure within the circuit board's circuitry, meaning the boiler can no longer verify if it is burning fuel safely or efficiently. As a result, the system shuts down to prevent any unpredictable behavior.
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 b4 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
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.
Affiliate links — we may earn a commission at no extra cost to you.