yep you will need the probit handset
just swapping out the pot on the steer angle wont work... if you change it it also needs to be calibrated which you will need the probit handset and the manual to run it through the calibration routine.

now as far as the self test goes, all that does is test the circuit and if it is pulling the contactor in during self test the circuit from the controller to the contactor is good. It sounds to me like you have another input issue elsewhere.
Get the probit handset and use that to monitor the operation, if there is a fault it will show up and help you locate where the problem is.