Vaillant 103430 Sensor NTC, DHW heat exchanger

The Vaillant 103430 is an NTC (negative temperature coefficient) sensor supplied for the domestic hot water heat exchanger on Vaillant appliances. It is a temperature-sensing component: its electrical resistance changes as the water temperature around it rises or falls, and the control board reads that changing resistance to work out how hot the domestic hot water actually is.

That reading drives the boiler's modulation and its safety logic, so a sensor that drifts or fails will show up as erratic hot water temperature or as a fault code rather than as an obvious physical break.

• Domestic hot water runs hot and cold instead of holding a steady temperature
• The boiler displays a temperature-sensor or hot water fault code
• Hot water temperature reads noticeably higher or lower than the setpoint
• The appliance cycles or locks out during a draw-off of hot water
• Intermittent hot water faults that clear on reset and return later

• This part is fitted to a gas appliance; installation must be carried out by a Gas Safe registered engineer.
• Confirm the part number against your appliance's parts list, and check the serial number and GC number where one is shown on the data badge.
• Inspect the sensor seat and any sealing washer or O-ring at the same time, as these are often disturbed when the sensor is removed.
• Check the wiring plug and loom for corrosion or a loose connection before condemning the sensor itself.