Boiler overheats and locks out; may display high temperature and shut down quickly after starting.

Safety: Turn off power and gas. Allow the boiler to cool completely before touching any components. Overheat conditions can cause scalding and damage.

Steps:

• Bleed air: Bleed radiators and any automatic air vents near the boiler. Airlocks can cause local overheating.
• Check system pressure: Ensure the pressure gauge reads within the Bosch‑specified range (typically 1.0–1.5 bar when cold). Top up via the filling loop if low, following the manual.
• Verify pump operation: When calling for heat, listen and feel for the circulation pump running. If the pump is silent or very hot to touch, it may be seized or failed.
• Inspect filters: Check any system filter or strainer on the return for blockage and clean if necessary.
• Reset STB: Some Bosch models have a manual reset overheat thermostat on the flow pipe or heat exchanger. Once cooled, press the reset button if accessible.

Professional tasks: Descaling or replacing a blocked primary heat exchanger and replacing a failed pump or STB should be done by a qualified heating engineer.