E9 224
Worcester Bosch
Safety Temperature Limiter (STL) Lockout
Boiler locks out with E9 224, often after overheating, may require manual reset.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Insufficient circulation due to blocked system, Pump failure, Airlock in primary circuit, Faulty STL device
How to Fix / Troubleshooting
Safety first: Allow boiler to cool fully. Do not repeatedly reset without finding the cause of overheating.
User checks:
- Check system pressure and top up to 1.0–1.5 bar when cold.
- Bleed radiators to remove trapped air.
- Ensure all valves on the heating circuit are open.
Engineer checks:
- Verify pump operation and free rotation; replace pump head if seized or weak.
- Check for sludge or blockage in the primary heat exchanger and system pipework; powerflush if necessary.
- Test the STL / overheat thermostat for correct operation and replace if faulty.
- Confirm correct bypass arrangement to maintain minimum flow through the boiler.
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Repair Difficulty
Hard
4/5
Required Part
Safety Temperature Limiter (Overheat Thermostat)
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