Vaillant 252805 Sensor - NTC

The Vaillant 252805 is an NTC sensor used across the Ecomax, Thermocompact VU and Turbomax Pro E appliance families. An NTC (negative temperature coefficient) sensor alters its resistance as temperature rises, so the boiler's control board receives a continuous, proportional temperature signal from the circuit it monitors. The board uses that signal to modulate the burner, manage pump overrun and shut the appliance down when the reading moves outside safe operating limits.

• The boiler locks out or fails to fire, often with a temperature-related fault code on the display
• Domestic hot water runs hot then cold, or settles well away from the set point
• Radiators or the cylinder heat erratically, with the burner short cycling
• A resistance reading taken across the sensor falls outside the expected curve for the ambient temperature
• Faults appear once the appliance warms up and clear again as it cools

• Match part number 252805 against the number on the existing sensor or in the appliance parts list.
• Check the model and serial plate, as the Ecomax, Thermocompact VU and Turbomax Pro E ranges use more than one sensor variant.
• Inspect the sealing washer or O-ring at the sensor pocket and renew it if it is compressed or perished.