20 ohms or kohms though? Those settings on a multimeter just determine the range it'll read up to in that setting, so if it's in 20ohms mode it'll read up to 20ohms and above that will just display something like "1" or "err" or some kind of out of range message. 20kohms means up to 20,000ohms. But obviously if you are wanting to test something you expect to have a reading of something like 10ohms then putting it in 20kohms mode means it'll be to small to register.
The ECU doesn't come into the test I'm talking about, unplug it and just test the resistance between the ecu connector and the sensor connector with the sensor not plugged in. Your just testing the wire between the connectors for breaks or dodgy connections. The resistance should be close to zero but any wire will have a small resistance in it, set the multimeter to the lowest range setting which is probably 20ohms, if I remember correctly you may see one or one and a half ohms there. Any larger resistances mean the loom is dodgy. Can either strip it down and repair/replace worn out looking wires and connectors or replace it but to be honest I'd still want to test a replacement loom as it'll be just as old and used. 15+ years of use, high under bonnet temps, oil contamination, fuel contamination etc... doesn't do looms any good.