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A Buick excel 1.8 car with a mileage of about 171 thousand km. User response: the engine coolant temperature is high and the engine fault light is lit. The maintenance process: 1) the connection Tech-2 measured three fault codes: the P0135 oxygen sensor heater circuit sensor 1, the P0141 oxygen sensor heater circuit efficiency group 1 sensor 2, the P0443 EVAP carbon tank discharge electromagnetic valve control circuit. The three fault codes will appear immediately after the fault code is cleared. 2) Tech-2 can not drive high speed and low speed cooling fans. The engine test data showed that the voltage of the oxygen sensor 1 remained unchanged for 690mV, and the voltage changed to 0mV after rapid acceleration. 3) when examining the failure of the cooling system, EF21 fuse fuse after the ignition relay is found, and the fuse is burned immediately after replacing the fuse (indicating a short circuit in the line). 4) after dismantling the attachment junction box, there are slight ablative marks on the cylindrical iron frame under the junction box, and the wire harness is checked one by one. It is found that the wiring harness of the junction box is worn out by the edge of the cylindrical iron frame, causing the grounding short circuit. Troubleshooting: repair damaged wiring harness and wrap the edge of cylindrical iron bar with adhesive tape to remove the trouble code, and the trouble code no longer appears. Tech-2 can drive high and low speed fans, troubleshooting. Line description: the heating power line of the oxygen sensor, the power line of the EVAP and the power line of the cooling fan relay before and after the EF21 fuse control, so the three fault codes in the cost case will be created after the fuse fuse of the EF21 fuse.