Being the expert mechanic that I am
, I'm just going to throw out a wild idea. Could you just take the hoses off of the steering cylinders and swap ends? If the problem is control- it should throw it all to the left, and if its a cylinder issue, things should still be messed up in the front end and staying to the right. I. E. on the steering cylinders swap rod for piston sides by switching the hoses and see what it does.
Nige is the expert here on anything CAT, but it would at least narrow it down from if you have a issue up front, vs problems with the electrical control. Of course I could be full of nonsense and nothing would work right on the machine because there's a bunch of sensors that won't let it do that.
I guess you could also put in some pressure gauges on the front between hose and cylinder and see if its putting any pressure out at the hose when the control is telling it to turn. You can also remove non pressure side hose on the "turn left" and see if you have hydraulic bypassing the piston of the cylinder, because of damage in the barrel when its at full right and you are telling it to go left. If there's hydraulic coming out of that return side, and the cylinder isn't moving, there's damage inside.
And just because I'm curious- in the steering diagnostic and set up that you attached nige- if the cylinder fell out of the range allowed for movement right- I. E. the cylinder has gone further to the right than the parameters of the system allows, would that then lock the steering and not allow steering in either direction because there is a fault and its out of the range- and then the system shuts off?