BoilerFaultCodesIdeal 4.2 Fault Code
Also written as 42 — same fault.
The Ideal 4.2 fault code means: your boiler has shut down because its internal temperature has become too high, often caused by a lack of water flow or a blockage.. It appears on 5 Ideal models in our database.
Technical description: Overheat Lockout
What causes the Ideal 4.2 fault?
This fault occurs when the water inside the boiler gets too hot, triggering a safety sensor to shut the system down to prevent damage. The most common reasons are a pump failure that stops water from moving or a buildup of sludge and debris that creates a blockage in the pipework.
This issue is frequently seen when homeowners turn their heating back on for the first time in autumn, as pump central shafts can seize up after being idle during the summer months.
DIY fix possible
Check that your boiler pressure is between 1 and 1.5 bar on the gaugeEnsure all radiator valves and the isolation valves under the boiler are fully openPress the 'Restart' button on the front control panel
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Gas Safe Register →Ideal models that show the 4.2 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 4.2 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