BoilerFaultCodesYour boiler is confused because its temperature sensors are giving conflicting readings or are installed incorrectly, causing the system to shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow/return temperature sensor fault
What causes this fault?
This fault occurs when the boiler's internal computer receives conflicting information from the two sensors that measure water temperature entering and leaving the unit. It is usually caused by a sensor that has developed a chemical fault, or because the sensors have been physically swapped or installed on the wrong pipes. Because the boiler cannot accurately track how heat is moving through the system, it shuts down to prevent the primary heat exchanger from 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 F.84 — 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.