Boiler displays A3 and shuts down; may be accompanied by overheating or rapid temperature rise

Safety: Turn off power and allow the boiler to cool completely. Do not reset repeatedly without finding the cause of overheating.

• 1. Check system pressure: Verify that the system pressure on the Buderus pressure gauge is within the recommended range (typically 1.0–1.5 bar when cold). Refill if low, following the manual.
• 2. Bleed radiators and boiler: Air in the system can cause poor circulation. Bleed radiators and any automatic air vents near the boiler. Top up pressure as needed.
• 3. Check circulation pump: Listen for pump operation when there is a heat demand. If silent or excessively noisy, the pump may be seized or air-locked. Carefully loosen the pump vent screw (if fitted) to release trapped air, with a cloth ready for small leaks.
• 4. Inspect filters and strainers: Many Buderus systems have a dirt separator or Y-strainer on the return. Clean any sludge or debris that may be restricting flow.
• 5. Reset STB: Some Buderus boilers have a manual reset button on the safety temperature limiter (often behind the front panel). Once the boiler is cool and circulation issues are addressed, press the reset button.
• 6. Check wiring and STB sensor: If the STB trips repeatedly despite good circulation, the limiter or its wiring may be faulty and should be tested and replaced by a technician.

Warning: Persistent overheat faults can damage the heat exchanger. If you cannot restore normal circulation, contact a qualified Buderus service engineer.