The Volvo troubleshooting manual for Bendix/Regina says that the code 2-2-1 will be set if the sense line is high or low for 3 minutes. Or if the voltage difference between high and low is less than .3 volts for longer than 10 seconds. These are pretty wide parameters and should only exist if there is a broken or bad connection or a really faulty sensor. The generic sensor swings a normal amount, and the "check engine" light has gone off since I probed the firewall connector, indicating that the fault condition has been corrected. I have to assume there is a loose connection at or near the locking connector and that the generic sensor will work ok with the car.
-WF