Fault phenomenon: a 2012 Jingcheng car, the mileage of the vehicle is about 19000km. User description: the vehicle's navigation function on the highway can not be used. The road test verifies that the cruise is on the cruise, the cruise lights will go off immediately and the cruise will not work, and the transmission capacity of the ECM DTC P2638 00 transmission is poor. Fault diagnosis: check the maintenance manual without this DTC diagnostic strategy. According to the inspection of cruising system fault strategy, there is no exception to the route measurement of cruise system, the ECM is programmed to match the test car, the cruising indicator light is not bright but the system still works when the cruise is on the cruise, and the DTC P2638 reported before is not displayed. Replacement of cruise switches and ECM modules failed.