NTC flakey

I'm using a NTC 502 thermistor ( like picture ) to kid my chilli seeds it is spring ( a propagator thermostat ) the other day it failed to respond to a low temperature , i put it on the bench and it seemed to work with ice and the soldering iron .. do they fail like this ? and should i swap it out ?

Historically thermistors are EXTREMELY reliable, and generally only die from physical damage, or by severe overheating.

I would imagine there's more likelihood of something else causing the problem? - but if you've got a spare, then try replacing it.

BTW, I see it says 103 on the side, is that a 10K thermistor then? - which is a standard value used in battery packs for monitoring the temperature during charging, but usually has stripes rather than numbers.

Agree with Nigel, NTCs are usually very reliable, have used them in aquariums for many years.
If the NTC measures ok with an ohm meter, then what about the rest of the circuitry or software if used ?

Usually place the NTC in a sheath of heat shrink tubing or seal with silicone compound to prevent any moisture affecting it.

These days we use the encapsuled DS18B20 sensors in the greenhouse and aquarium , they come with the cable attached and sealed with epoxy.

I'm now thinking it was not sealed that well to the cable , It started working as i attempted to measured it ( Its a 5k device ) so I will construct another with a better sealed connection . thanks