BoilerFaultCodesYour boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, which means it cannot heat your home or water safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow Thermistor Fault
What causes this fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors the heat of the water leaving the unit. It is usually caused by the internal components of the sensor failing over time or a loose connection in the electrical wiring that links the sensor to the main computer.
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 F04 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.