A vehicle is hard to steer to the left only. What is the most likely cause?

When a vehicle is hard to steer to the left specifically, it indicates that there may be an issue within the steering system that is hindering its smooth operation on one side. Worn power steering gear is the most likely cause in this scenario because it directly affects the ability to turn the steering wheel. If the power steering gear is worn, it may not provide consistent assistance when turning the wheel in one direction as opposed to the other, leading to increased resistance and difficulty in steering.

A weak power steering pump could cause overall steering difficulty, but it typically would not create a directional disparity like steering hard to one side. In contrast, a tight steering shaft universal joint may restrict movement equally in both directions, so it wouldn’t necessarily manifest as an issue only while steering to the left. A binding ball joint can lead to handling problems as well, but again it would likely affect the vehicle's overall steering behavior rather than being specific to one direction. Thus, worn power steering gear produces the symptoms described best, making it the most plausible explanation for the problem at hand.