A 2004 Volkswagen POLO1.6 manual car with a mileage of about 180 thousand km, equipped with BCD engine and 5 gear manual transmission. User response: the car has not been running on the air conditioner for a long time, and the cooling fan is in a high speed gear, and the low speed gear has not been running. Inspection and analysis: the maintenance personnel changed the fan control unit J293, but the test failed. Enter the air conditioning control unit J301 query, there is fan V154 fault memory, but this is not directly related to the fan. When reading the data, the system pressure is 0.9 MPa when the air conditioner is not running, corresponding to the current ambient temperature of 37 degrees, slightly higher. Users say that the performance of air-conditioning has been better since last year, but the noise of high speed fans has often been heard recently. Air conditioning, about 30 s, the fan high-speed rotation, read the measurement value, the fan high-speed gear in high pressure up to 1.6 MPa start, about 1.2 MPa when closed, basically in line with the cooling fan running law. To shut down the air conditioner and to increase the coolant temperature of the engine, observe the low speed running of the fan. However, the temperature is always at 93 degrees, and the temperature control switch of Zuo Sen pulled down, which crossover the analog signal input of the low speed block terminal, and the fan low speed gear is not running. Pull out the fan connector, connect the connector intermediate terminal (low speed) with the battery, and do the electrifying test. It is found that the low speed gears of the 2 fans do not work. The analysis shows that because the cooling fan does not work at low speed, the pressure of the system increases rapidly, which results in the high speed operation of the fan. Troubleshooting: replace cooling fan and troubleshooting.