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This is the PTC (positive temperature coefficient sensor), as opposed to the later 1992 1/2 NTC (negative temperature coefficient sensor). With a PTC sensor the resistance increases as the temperature increases. One way to confirm it is a PTC sensor is that it has a 1/8 NPTF thread. The NTC is the opposite - as temperature increases resistance decreases - NTC sensors have a M10X1.25 thread - the difference in thread was intentional by Volvo so that people would not mix the sensors up since they require different ECUs and ICU EZK units. This PTC sensor is nearly impossible to find and NTC sensors are currently easily available.
We need to find a way to repair these early PTC (positive temperature coefficient) sensors ourselves. PTC thermistors of a variety of types are apparently manufactured, and theoretically we could open up the bad sensor and drop in a new $5 thermistor then reseal it.
I am currently searching for the same sensor or a replacement thermistor. My 1991 240 has a chronic 4-1-3 EGR sensor temp. warning and I am in the process of understanding the EGR system. In 1992 1/2 Volvo upgraded to a NTC sensor (negative temperature coefficient) that uses a different ECU and ICU EZK. I am currently trying to understand if I can update my 1991 to use the 1993 system by swapping the ECU and ICU EZK - the advantage is that the NTC sensors are easily available. So my working theory is that with a 1993 ECU and ICU EZK I could use a NTC sensor and everything would be fine... however, I haven't tried that yet. That is the next thing I'll try if I can't find a functional PTC sensor or replacement thermistor.
Also, someone might ask me why bother with EGR? I'm sick of the check engine light and there is a chance this will need to be smogged in CA in the future, so I'm trying to sort it out in advance.
If anyone reading this has any experience with thermistors, I'm trying to learn more about those and how to repair a broken sensor.
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