Why would a technician not use the open end of a wrench to untighten a frozen nut or bolt on a vehicle?

When a technician is faced with a frozen nut or bolt, using an open end of a wrench may result in rounding the fastener. The open-ended portion of a wrench engages with only two sides of the nut or bolt, which provides less grip and contact area compared to a box-end wrench that wraps around the entire hex. This limited contact can lead to slippage, especially when a significant amount of force is required to break the bond of a frozen fastener.

As the technician applies force, the open end can easily slip off, and if enough pressure is applied, it can cause the corners of the fastener to round off. Once the fastener is rounded, it becomes exceedingly difficult to grasp with any tool, making removal even more challenging. Therefore, using a box-end wrench or a socket, which engages all sides of the fastener, is a better approach for reducing the risk of rounding and successfully loosening a stuck nut or bolt.