Domestic hot water temperature fluctuates between hot and cold during use on a combi boiler, with no error code.

Safety first: Water can suddenly become very hot; be cautious when testing taps or showers.

• Check flow rate: Try reducing the flow at the tap; very high flow can exceed the boiler's capacity and cause fluctuations.
• Test multiple outlets: See if the issue occurs at all taps or just one (which may indicate a local mixer problem).
• Engineer checks:
  • Inspect and descale or replace the plate heat exchanger if restricted.
  • Test the DHW NTC sensor and replace if readings are unstable.
  • Clean or replace the DHW flow sensor/turbine.
  • Check gas valve modulation and combustion settings.