Boiler shows 103, frequent lockouts, radiators only warm at top or not heating fully

Safety: Turn off power and allow the boiler to cool before touching pipework.

• Check all radiator valves: Ensure TRVs and lockshield valves are open to allow circulation. A closed circuit can cause overheat and error 103.
• Bleed radiators: Remove trapped air from all radiators. Top up system pressure as needed.
• Check pump speed and operation: On the Ariston circulation pump, verify the speed setting (if adjustable) is not on the lowest setting for a large system. Listen for unusual noises or grinding that indicate pump wear.
• Feel flow and return temperatures: With heating demand, carefully touch (briefly) the flow and return pipes. If flow is extremely hot and return is very cool, circulation is poor, suggesting blockage or pump failure.
• System cleaning: Add a central heating cleaner via a radiator or dosing point and run the system as per product instructions, then flush and refill with inhibitor. Severe sludge may require professional power flushing.
• Reset boiler: After improving circulation, reset the boiler. Persistent 103 faults require an engineer to check the primary heat exchanger and pump performance.