Temperature sensors and heaters belong in the diagnostics subsystem of the LISA Technology Package (LTP) on board LISA Pathfinder, the technology demonstrator for LISA. A number of these diagnostics items are placed at short distances from the LTP proof masses, and are negative temperature coefficient (NTC) thermistors. By design, these devices have tiny amounts of ferromagnetic materials which therefore constitute a potential source of disturbance to the performance of the LTP. We present a detailed magnetic characterisation of the NTC's, and use the data to evaluate their impact on the acceleration noise budget of the LTP. The effect is seen to be small, and can be further reduced if the NTC's are submitted to a demagnetisation process before they are attached. Re-magnetisation is unlikely, as rather strong fields (mili-Tesla) are required to re-magnetise the NTC's.