The units are said to be giving a poor ride and that noises are coming from the underside.
When class 158's were first introduced they suffered identical problems which resulted in more substantial yaw damper mounting brackets having to be fitted, which was a major exercise.
CAF 331s and 397s and also Caledonian Sleeper and TPE Mark 5 loco-hauled stock.which are of similar design could also be affected.
A yaw damper is a transverse mounted shock absorber, used to prevent railcars and locomotives from swaying excessively from side to side. Yaw dampers prevent rail vehicles from striking station platforms and reduce the gap that must be left between the vehicle and the platform.