Boiler indicates an issue with the hot water sensor or control; DHW may not reach set temperature or may fluctuate.

Safety: Isolate electrical power and close the gas valve before opening the boiler casing.

Diagnostic steps:

• Check DHW setpoint: Verify the DHW temperature setting on the Bosch user interface. Ensure it is not set too low or in ECO mode if higher temperature is desired.
• Inspect DHW NTC: Locate the DHW NTC on the hot water outlet or plate heat exchanger. Check for secure mounting and good thermal contact. Loose sensors can cause erratic readings.
• Measure resistance: With power off, disconnect the NTC and measure resistance. Compare to Bosch spec at room temperature. Replace if out of range.
• Check harness and plug: Inspect the connector at the sensor and at the PCB for bent pins, corrosion, or broken conductors. Repair or replace harness as needed.
• Parameter reset: If available on your model, enter installer/service menu and reset DHW parameters to factory defaults (refer to Bosch installation manual). Do not change gas settings.

Professional help: If the sensor and wiring are correct and the fault persists, the main control board may be defective and should be tested and replaced by a qualified Bosch technician.