This model has self diagnostics that flashes code number four. That says I have a bad "Shift Mode Coil". What is that, where is it and how do I replace it?

The symptoms are that the whole tub swishes back and forth but the agitator seems locked to the tub. It never goes into agitate mode and the whole machine clicks and bangs as it tries to reverse the tub direction. It will swish one way for about a second or two then reverse direction. It does that during the entire wash, no matter which cycle I choose.

Seems to me it is not "Shifting modes". Why?