BoilerFaultCodesYour system has a faulty temperature sensor in the main pipework, which means it cannot accurately monitor the water heat flowing through your home.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Header Thermistor (T1) Error – for cascade and hybrid
What causes this fault?
This fault occurs when the main temperature sensor responsible for monitoring the shared water supply becomes unresponsive or provides an impossible reading. The most common reason is a physical failure of the internal sensing element, though it can also be triggered by loose or corroded electrical connections between the sensor and the main control board.
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 9.1.5 — 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.