BoilerFaultCodesIdeal 42 Fault Code
The Ideal 42 fault code means: your boiler is producing more heat than your hot water tank can safely handle, causing the system to stop for protection.. It appears on 2 Ideal models in our database.
Technical description: The hot water tank is too small for this unit power (external hot water tank).
What causes the Ideal 42 fault?
This fault occurs when the boiler generates heat faster than your hot water cylinder can absorb it, leading to a rapid temperature spike that triggers a safety shutdown. The most common reason is that the boiler's power output is too high for the size of your external water tank, or there is a restriction preventing the heat from transferring efficiently into the water.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Ideal models that show the 42 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 42 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