Cause
Low power steering fluid level
Not enough fluid in a hydraulic power steering system, so the pump cannot maintain pressure. The pump depends on fluid for cooling and lubrication as well as for pressure, so running it low destroys it — which is why this matters beyond the steering feel.
Aeration is the part that produces the odd symptoms. A level low enough to let the pump draw air gives a system that whines, feels lumpy, and is worse on lock and when cold — and the behaviour changes with cornering and gradient as fluid moves in the reservoir. Foam in the reservoir is the confirming evidence.
These are sealed systems, so a low level always means a leak. Topping up without finding it means meeting the same fault again with a pump that has been damaged in the meantime. Check the rack gaiters, the hose crimps and the pump's shaft seal, and use the specified fluid — many systems require a particular type, and mixing them degrades the seals that were holding.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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