Boiler locks out with A03 shortly after starting; may be accompanied by very hot return pipe and no heat to radiators.

Safety first: Isolate electrical power and allow the boiler to cool. Do not repeatedly reset an overheating boiler.

• Check radiator valves: Ensure at least a few radiators are fully open; all TRVs closed can cause poor circulation and overheating.
• Check system pressure: Confirm pressure is within the recommended range (typically 1.0–1.5 bar cold). Re-pressurise if needed.
• Reset once: After cooling, perform a single reset. If A03 returns, stop using the boiler and call an engineer.
• Engineer checks:
  • Verify pump speed and operation; replace if not circulating properly.
  • Inspect and clean or replace the primary heat exchanger if restricted.
  • Test the safety thermostat and NTC sensors with a multimeter.
  • Check for closed isolation valves on the flow/return under the boiler.