E04
FAULT CODE
Alpha
High Limit Thermostat Tripped
High Severity
Engineer Required
The high-limit (STB) safety thermostat has activated due to excessive temperature, cutting the gas supply.
Step-by-Step Fix Guide
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Step 1
Do not repeatedly reset
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Step 2
Turn off the boiler
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Step 3
Allow to cool for at least 30 minutes
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Step 4 Engineer only
Call a Gas Safe engineer to diagnose the root cause
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.