[M.A.C]

My '05 STS has the same problem. The other day I noticed it was harder turn when pulling into a parking spot. Now I can't turn unless moving and accelerating, which must increase line pressure enough. I did notice the power steering fluid was a little low so I added fluid. I'll check for leaks but I have a feeling that the power steering pump has to be replaced.

 

[M.A.C]

I noticed dirt inside when I topped off with power steering fluid. So I'm thinking the fluid is worn out and dirty. I might try to flush the fluid by disconnecting the hose and starting the engine and let it pump out into a container. The pump is not noisy, so I'm thinking the fluid may be the problem?!