BoilerFaultCodesYour boiler has detected a safety fault with the temperature sensors or internal wiring that prevents it from firing up.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Fault code 20 shows (from Fault Finding Test B)
What causes this fault?
This fault occurs when the boiler's control board detects that the temperature sensors are sending an illogical signal or have disconnected entirely. It is usually caused by a physical failure of the internal thermistor or damage to the delicate wiring that connects these sensors to the main computer. Because the boiler can no longer accurately monitor how hot the water is, it shuts down as a safety precaution to prevent overheating.
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 20 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Vokera Synergy E 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.