Many modern engines use a computer controlled mechanical system known as Variable-Valve Timing (VVT) to adjust engine valve timing based on operating conditions. Vehicles equipped with a V-6 engine and VVT are being recalled because bolts securing components of the VVT system could become loose due to normal impacts generated immediately after starting the engine when cold. The loose bolts could cause the VVT gear housing and sprocket to separate. This would cause the engine to stop while the vehicle is being driven, increasing the risk of a crash. Lexus dealers will replace the VVT unit on affected engines to correct this concern.