BoilerFaultCodesYour boiler has lost communication with a sensor that monitors the temperature of the water flowing into your heating system, usually because of a loose or damaged wire.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Controls without flow temperature sensor (low loss header) - System flow temperature sensor lead break
What causes this fault?
This fault occurs when the boiler's control board can no longer detect a signal from the temperature sensor located on the main pipework. The most common reason is a physical break in the internal wiring or a sensor that has failed due to age, preventing the boiler from knowing how hot the water is as it leaves the unit. Because the system cannot safely monitor the heat, it shuts down to prevent damage.
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 28 — 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.