A 2002 annual production of Passat B51.8T sedan, mileage of 80,000 km. After the summer air conditioning is sometimes working properly, and occasionally in the driving there is no cooling and the phenomenon of hot air, no matter how the owner adjust the air conditioning related buttons do not work, observe the air conditioning control panel shows "-50 ~ 40 ℃". This phenomenon does not often occur, there will be half an hour after the rule and more and more.
    Maintenance personnel to receive this car, the first with VAG1552 into the 08 air conditioning system, with the function keys 02 read the fault code, the results show "00779", that is, the external temperature sensor G17 (position in the front bumper outlet, condenser before) failure. According to this clue, combined with the circuit diagram, check the external temperature sensor G17, the plug connected well. Measure the resistance with a multimeter is 395Ω, with the maintenance manual control, in the normal range. But also check the cooling fan and coolant and other related factors, were not found abnormal. Taking into account the occasional occurrence of the vehicle air conditioning failure phenomenon, suspected external temperature sensor thermal components in high temperature or other special state performance instability, decided to replace the external temperature sensor G17. After the function key 05 code, the test did not find a few times to reproduce the fault, so the car. Who knows for a week, the old problems have emerged.
    This car into the maintenance station, the maintenance staff and V.A.G1552 query, the last code as the "00779". This strange, obviously the detector shows the point of failure, the relevant parts and no exception, why can not rule out the fault? Also check the air conditioning and related systems. The air conditioning control unit will not always have a problem? After the new pieces, with V.A.G1552 code, into the 08 air conditioning system, with the function keys 07 code, enter 05000. And then set, with the function key 04 input 063. After the completion of these work, start the vehicle test, air conditioning work well, in order to verify whether the fault is to explain to the customer after the maintenance situation in the use of observation of air conditioning operation. And after more than a week, the fault again appeared, indicating that the fault point still did not find.
    When the car again, the maintenance staff once again with V.A.G1552 done a test, the fault code has not changed. So that people scratching their heads, check the information, turn the manual can not find the clue, then think of each failure when the air conditioning control panel appears "-50 ~ 40 ℃", this figure should be in front of the car temperature display, followed by the car Set the temperature display. When there is minus degrees Celsius, air conditioning, of course, will not work cooling. And the perception of the temperature in addition to the outside temperature sensor, there are indoor temperature sensor.
    During the inspection of the sensor in the control manual, it was found that the fresh air intake air temperature sensor G89 controls the outside temperature and, when it fails, is replaced by the external temperature sensor G17. At this point suddenly, air conditioning failure is likely to be caused by G89 problems. Fresh air intake air temperature sensor G89 in the right front seat after the debris box, close to the evaporator behind the air inlet. For a new sensor, the repeated test, the failure did not appear, so the fault completely ruled out.
    Why fault diagnosis instrument can not accurately find the fault point? In fact, G89, G17 these two sensors are aware of the outside temperature, mainly to G89. When G89 fails, G17 works as an alternative sensor. As the maintenance personnel used in the detection process is not the original fault diagnosis instrument, so only the detection of external temperature sensor G17 fault code, and the main cause of the fault hidden. Take this as an example, hope that peer problems when thinking, think, test equipment is just a guide to our thinking and troubleshooting a tool, do not fully believe that the detection equipment, especially the use of non-original diagnostic equipment should be doubled cautious.