BoilerFaultCodesIdeal 4.5 Fault Code
Also written as 45 — same fault.
The Ideal 4.5 fault code means: your boiler has overheated to a critical level and a safety component has permanently cut the power to prevent serious damage.. It appears on 8 Ideal models in our database.
Technical description: THERMAL FUSE LOCKOUT
What causes the Ideal 4.5 fault?
This fault occurs when the internal temperature inside the main heat exchanger has reached an extreme, dangerous limit, causing a one-time safety fuse to blow. The root cause is typically a severe lack of water flow or a significant build-up of debris and limescale that has caused the component to dry-fire and get too hot. Because the fuse is designed to break permanently to protect your home, the boiler cannot be restarted simply by pressing a button.
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 →Ideal models that show the 4.5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 4.5 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026