You can do your 4 way connector test to eliminate any other possibilities but make sure your battery voltage is up before you do anything else.
What Clay is saying is the the step extends and retracts until it reaches it's normal travel, then the current rises and the control unit senses this rise and shuts off the step operation.
Because yours is not shutting off in either direction it kind of points to the control unit being defective.
When you do testing from underneath remember not to position yourself (or your fingers) under the step or linkage in case it extends before you can move. Good luck and let us know what you find.