BoilerFaultCodesYour boiler has detected that a temperature sensor is not properly attached or is incorrectly mounted, preventing the system from heating up safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Mounting fault sensor S1
What causes this fault?
This fault occurs when the boiler's main flow temperature sensor has physically shifted or detached from the internal pipework. Because the sensor can no longer accurately measure how hot the water is, the system shuts down to prevent the boiler from overheating. This is often caused by a weakened retaining clip or vibration over time loosening the sensor's connection.
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 f015 — 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.