Boiler runs but hot water temperature is unstable or incorrect; DHW temperature on display does not match actual outlet temperature.

Safety first: Turn off the boiler at the main switch and isolate electrical power at the breaker. Allow the boiler to cool fully and close the gas isolation valve before removing any covers.

Steps to check and correct:

• Access the DHW NTC: Remove the front panel according to the Bosch service manual. Locate the domestic hot water NTC sensor on the plate heat exchanger or DHW outlet pipe.
• Inspect wiring: Check the NTC plug and harness for loose connections, corrosion, or damaged insulation. Reseat the connector firmly. Repair or replace damaged wiring.
• Test the sensor: Disconnect the NTC and measure its resistance with a multimeter. Compare the reading to the Bosch temperature–resistance chart (e.g., around 10 kΩ at 25°C, value varies by model). If the reading is open circuit, shorted, or far out of spec, replace the NTC.
• Check for scale: If the sensor is in a pocket on the DHW heat exchanger, remove and inspect for limescale. Descale or replace the sensor pocket/heat exchanger if heavily scaled.
• Reset and test: Restore power and gas, then reset the boiler. Run a hot water tap and verify that the displayed temperature is stable and close to the actual outlet temperature.

When to call a professional: If the NTC and wiring test good, the fault may be on the main control PCB. PCB diagnosis and replacement should be carried out by a Gas Safe / certified technician.