Brahma Immersion temperature sensor ST09 18200453

This is an immersion NTC temperature sensor with model designation ST09 and type NTC108. It measures the temperature of the heat transfer fluid in heating and hot water systems and transmits a continuous resistance signal to the boiler control board. The sensor has a G1/8 connection, a red connector, and is specified for version CH15. Its dimensions are 40 mm by 20 mm by 15 mm, with a net weight of 0.01 kg and a gross weight of 0.015 kg. It is intended for use in Vaillant equipment.

Sensors of this type are selected by matching the resistance characteristic (typically 10 kΩ or 12 kΩ at 25 °C) to the control board, as well as the probe form (immersion, clamp-on, or sleeve), cable length, and connector type. Before ordering, measure the pocket or pipe clamp dimensions to ensure the probe fits physically, and verify that the connector matches the board. Installation requires the probe to be inserted into the immersion pocket or attached to the pipe with good thermal contact.

Typical failure modes for NTC sensors include drifting resistance, broken cable strands, and poor thermal contact between the probe and the pipe. When a sensor fails, the boiler may overheat, underheat, or cycle erratically because the control board receives incorrect temperature readings. Check the sensor's resistance at a known temperature and inspect the cable and mounting for physical damage.