1 -Disconnect the switch wires on the PIR and short the NC/NO wires at the PIR end. Are the Loop values normal?
2 - remove the zone wires at the panel end and short them there. Are the loop values normal then?
If the values remain the same you might have a short in the wires going to the PIR. The PIR can also have an issue.
Here updated my very old PIRs to newer combo PIR Microwave sensors. I did not have issues with the old PIRs (for over 15 years).
You can disconnect both sides and check the wires with a VOM for any shorts if you want.
My loop values all read 147-8 when secure (PIRs and door contact sensors) and 253-255 when open. Have always soldered in place all of the EOL resistors. That is me and it was a PITA. I used a mini tiny torch to do this rather than a soldering iron.