A mileage of about 150 thousand km, chassis type W140, equipped with M119 engine and automatic transmission of Mercedes Benz S500 sedan. User response: the vehicle suddenly stalls and restarts the engine while the starter does not respond. Turn on the ignition switch. The dashboard shows the "starting error", indicating that the fault is related to the engine anti-theft lock. Check the ignition key, confirm that the car is equipped with DAS anti-theft system. The connection diagnostics is self diagnostic, and the engine system has a fault code. The content is "7, the DAS signal interruption of the engine control module". Clear the fault code, the fault code can not be removed. Check the data related to the DAS function and confirm that the engine is locked by the DAS system. Self diagnosis of DAS system results in failure of communication. Remove the dashboard and check the DAS control module. It is found that the DAS control module does not have a working power supply. Check the whole vehicle power supply line and fuse fuse 20 on the left fuse box of the engine room. Replace the new fuse, the new fuse immediately burned, indicating that there is a short-circuit fault on the line. Check the wires connected with fuse No. 20, and find the short wiring of the air quality sensor (the sensor of the air conditioning system, the air conditioner filter housing). Repair the wiring harness, replace the new fuse, start the vehicle, and the engine runs smoothly. There are two fault codes for self diagnosis of DAS system: "B170, incorrect starting signal of engine", "B1703, poor performance of transmitter". Clear these two memory fault codes and eliminate them thoroughly. The car adopts DAS2 electronic anti-theft system and has independent DAS control module. In this case, the fault is very special. The air quality sensor adopts the environmental protection line, which is easy to oxidize and short circuit, resulting in the power failure of the DAS control module and the engine can not start. Besides, in case of such trouble, the ignition key and DAS control module should be checked. In general, as long as the failure of the DAS system appears, the dashboard will display the fault information. At this time, the diagnostic instrument can be used to query the DAS data of the engine system and determine whether the DAS system is normal.