BoilerFaultCodesYour boiler has shut down because a sensor has detected that the exhaust gases are getting too hot or the sensor itself has failed.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Failed flue temperature – check operating system
What causes this fault?
This fault occurs when the internal sensor detects that the gases leaving your boiler are reaching dangerously high temperatures, triggering a safety shutdown. The most common cause is a buildup of debris or limescale inside the main heat exchanger, which prevents heat from being transferred to your water properly and instead sends it out through the flue. It can also be caused by a faulty sensor sending an incorrect temperature reading to the boiler's 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 30 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Ravenheat CSI 120 is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.