BoilerFaultCodesIdeal ERROR CODE 4 Fault Code
Also written as ERRORCODE4 — same fault.
The Ideal ERROR CODE 4 fault code means: your boiler is having trouble venting exhaust gases safely, which prevents it from firing up to provide heating or hot water.. It appears on 2 Ideal models in our database.
Technical description: APS / FAN ALARM
What causes the Ideal ERROR CODE 4 fault?
This fault occurs when the boiler's internal safety sensors detect that the fan isn't moving enough air to safely clear exhaust gases from the system. It is usually caused by a failing fan motor, a blockage in the air pressure tubes, or debris caught in the flue that prevents a steady flow of air. Without this confirmed airflow, the boiler won't ignite to ensure carbon monoxide doesn't build up inside the unit.
This fault is slightly more common in autumn and winter, as high winds can cause back-pressure in the flue or debris like fallen leaves can block the external air intake.
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 ERROR CODE 4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal ERROR CODE 4 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