PT1000 suddenly reads 42C for room temp hot end

[jens55]

A few days ago I replaced a thermistor on one of my printers with a PT1000 sensor. I adjusted config.g for the new sensor and everything worked just fine. A dozen or so prints follow and as far as I know, the temperature reading was always ok (reading within a degree or less of actual room temp when cold).
Today I noticed that my printer is reading 42C when it should be around 20C. I think the only reason I noticed this was because my thermostatically controlled lights where on when the print head was around 20C and the ligths are only supposed to come on above 40C for the hot end.
I power cycled the printer, wiggled all the connections, switched the config.g to a thermocouple (reads correctly) and then switched back to the PT1000 (temperature is off by around 20C). I tried manual calibration but this is a Duet2 board so that is not supported.
Next I tried M308 S1 and the duet reported a L parameter of 0 and an H parameter of 128. I twiddled the L parameter and found the temperature to be correct at L30. I put that into my config.g, rebooted and now my temperature is off again and it requires an L 50 to get the right temperature reading. I did this one more time and now L80 was the correct figure.
With an ambient temperature of 20C, I started out with a reading of 42 with L0 but later on it went to 29C at L0.
I can see that one might need to tweak parameters but this PT1000 was reading just fine when I first installed it so what gives? Why does the reading fluctuate and requires different L parameters to correct the room temperature reading. Shouldn't this be something that is set once and stays there?
Is there a common failure mode for a PT1000 that has it's temperature going all over the place?

[dc42]

@jens55 most likely either the PT1000 has developed a fault or you have a bad connection in the wiring between the PT1000 and the Duet, for example a bad crimp connection. A high resistance connection will cause a PT1000 to over-read.

If you have a 1K resistor to hand, you can check that the Duet is OK by connecting it to the Duet in place of the PT1000. It should give a reading close to 0C.

[jens55]

@dc42 Thanks. I will give the resistance measurement a go. My first thought was 'bad connection' but when jiggling all the cables did not affect the temp indication at all, I was not sure what was going on. Changing everything over to a thermocouple and getting the correct reading seemed to confirm that the wiring was ok. I will try and replace the sensor if the resistance measurement shows 0C with a 1000 ohm resistor connected in place of the PT1000.