Boiler shows E03 and shuts down shortly after starting, primary flow temperature rises rapidly

Safety: Allow the boiler to cool fully before touching any pipework. Isolate electrical power before removing covers.

User checks:

• Check system pressure is at least 1.0 bar when cold; top up via filling loop if low.
• Bleed air from radiators and any accessible manual air vents.
• Ensure all radiator valves and isolation valves near the boiler are open.

Engineer checks:

• Confirm the Keston circulation pump is running when there is a heat demand. Listen for operation and check for vibration; test for 230 V supply at the pump terminals.
• Inspect and clean the primary side of the Keston plate heat exchanger if blocked with sludge or scale; powerflush system if contamination is severe.
• Check for closed valves or blocked strainers on the flow/return pipework.
• Test the primary NTC sensor and overheat thermostat for correct resistance values and continuity; replace if out of specification.
• Verify correct pump speed setting and that bypass (if fitted) is adjusted correctly to maintain flow.