No announcement yet.

The G42 IAT How to Replace, Repair or Calibrate

  • Filter
  • Time
  • Show
Clear All
new posts

  • The G42 IAT How to Replace, Repair or Calibrate

    The G42 sensor part number 034905379B I am sure you are all aware is NLA. I cannot find a new replacement so I am looking to do the following.

    Check the ECU reading from about 20 to 40 deg cel and the sensor resistance at the same points. If its out then can I just calibrate it by adding a resistor in series or parrallel as needed.

    Can you buy a PTC replacement component and then calibrate it. Does anybody know where and what to buy as a suitable PTC sensor that can be soldered in.

    Twice recently I was out early in the morning and the outside temperature was below 18 deg cel which is cool for us and the car was running well no issues. As the day got warmer 28 deg cel the car was starting to drop in performance. I will also add that we do also get outside temperatures of 35 to 43 deg cel and the car does not perform. From going through the forum a thread advises at 40 deg IAT the ECU goes into a protection mode of some sorts and pulls back timing or boost. The outside temperature at 35 to 43 deg cel is obviously an issue in SA. But at 25 deg I do not think there should be any need to pull back timing or boost unless the sensor is causing an issue.

    My concern is the accuracy of my G42 sensor.

    It would be nice if the G42 sensor repair or replacement can be solved.

    Below is some of the data I found on the forum. I cannot confirm the accuracy of my info.

    G42 Datasheet (


    The G42 IAT is connected to the ECU via pin T55/44 with a blue striped brown wire (BR/BL). A digital multimeter can be used to measure the resistance of the two-pin device G42 intake air sensor with the connector harness removed. With the engine OFF long enough to cool sufficiently, at an ambient air temperature of 20 degrees, a value of 450-650 ohms should be measured across G42.

    The resistance should be approximately 500 ohms at 25C (70F). The resistance measured doesn't need to be "exactly" as shown, it just needs to be close to the value shown.

    Here is a graph of the sensor resistance versus temperature.

    Last edited by orsum; 28 May 2021, 08:38.

  • #2
    First check done.
    Temperature measured 22.2 cel resistance measured was 495 ohms.
    ECU reading using Ross-Tech 22,2 cel
    That seems ok

    Based on the type of sensor used if its right at 22,2 deg then I would think it will be right at all the points in its range. If I need to offset the range I dont think I will be able to do it without messing with temperature at 22,2 deg

    Now to test if it is accurate between 22,2 and 40 cel

    Going to try to make a warm box to put the sensor and my Fluke temperature probe in to see if they read the same.

    Once I have proved the sensor I am going to log a trend on a warm day run. (doubtful I will get a 40 plus day in out winter)
    Last edited by orsum; 28 May 2021, 09:19.


    • #3
      If your G42 is giving you a believable reading at a static temperature then it's highly unlikely there's anything wrong with it. True, the IAT reads the inlet air temperature but as it's doing it after the turbo and intercooler the air temperature it reads won't be directly linked to the outside air temperature at the time, it'll always be higher depending on how much work your intercooler is having to do.

      If you go for a drive with VAGcom connected you'll see just how much and how quickly the G42 reading changes according to how hard you're driving the car.

      There are plenty of sensors out there, if you go to an electronic components site like you'll be able to find a thermistor whose characteristics match G42's but the challenge is that you want an unencapsulated one so it can react really quickly to changes in temperature. An easier route could be to use the IAT sensor from a later Audi/VW with a turbo but it's not something I've looked into. You can't "calibrate' a PTC as such, all you can do is modify its resistance by adding a fixed series or shunt resistor.

      No question G42 is very important to the ECU as one of the inputs controlling boost but I believe not in the hard way you imply at 40 degrees. Available boost will be progressively limited as IAT rises.

      I'd be more concerned over a blocked intercooler than a faulty G42.