A03
Wolf
Safety Temperature Limiter (STB) Tripped / Overheat
Boiler shows A03 and shuts down, overheat symbol may be displayed
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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
Blocked or closed heating circuit valves, Failed or seized circulation pump, Air lock in primary heat exchanger, Faulty safety temperature limiter or wiring
How to Fix / Troubleshooting
Safety: Turn off power and allow the boiler to cool fully before touching any pipework. Do not bypass safety devices.
- Check system pressure: On the pressure gauge, ensure the cold pressure is typically between 1.0–1.5 bar (refer to Wolf manual for exact range). If below, top up via the filling loop following the manufacturer instructions.
- Verify radiator valves: Ensure that at least several radiators are open. If all TRVs and isolation valves are closed, water cannot circulate and the boiler will overheat.
- Bleed air from radiators: Use a radiator key to release trapped air starting from the highest radiators. Re-pressurise the system as needed.
- Reset the boiler: Once pressure and valves are correct, reset the boiler. Listen for the circulation pump starting. If the pump is silent or very hot to touch, it may be seized.
- Call a technician if A03 recurs: They will test the STB sensor, check pump operation and flow, and inspect for sludge or blockages in the primary circuit.
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Repair Difficulty
Hard
4/5
Required Part
Safety Temperature Limiter (STB)
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