gday. ive serviced hundreds of truncheons in my time.
spring should be in the top only. check the battery wires. they alsways get crunched putting the top back on.
they should never die.
clean graphite probe with non-scented jif and water. its slightly abrasive and removes a layer. oily shit fucks with the readings.
they are factory calibrated but can go out over time. as good as they are its a bitch to send em back for a service.
always use good baterries.check yr bayyery terminals arent corroded too.
hope this helps.