Which of the following is the least likely cause of an out-of-spec caster on a unibody vehicle?

A bent idler arm is the least likely cause of an out-of-spec caster on a unibody vehicle because the idler arm primarily affects the steering linkage rather than the suspension's alignment angles, such as caster. Caster is determined by the orientation of the strut or control arm in relation to the steering axis. Since the idler arm's function is to maintain the position of the center link in a steering mechanism, its distortion generally influences toe and possibly other steering angles, but not the caster itself.

In contrast, components like the crossmember, lower control arm, and strut rod are directly linked to the vehicle's suspension geometry, which governs alignment settings like caster. A misaligned crossmember could easily affect the positioning of the suspension components, altering the caster angle. Similarly, a damaged lower control arm can directly change the angle at which the wheels are set relative to the vehicle's frame, affecting caster. A damaged strut rod provides additional support to control arm positioning, and its impairment can also lead to caster misalignment. Thus, a bent idler arm is the least impactful on caster specifications.