BoilerFaultCodesYour boiler has detected that a vital temperature sensor is incorrectly fitted or has become detached, preventing the system from monitoring heat levels 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 S3
What causes this fault?
This fault occurs when the S3 sensor, which monitors the temperature within the heat exchanger, is not properly seated against the pipework or has physically detached. The most common reason is a loose or damaged retaining clip, but it can also be triggered by a faulty wiring connection that prevents the boiler's computer from receiving an accurate reading.
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 f016 — 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.