Scan your VW computer for trouble codes using inexpensive OBDII VAG-COM USB cable.

The check engine light came on my 98 GTI vr6 and I don’t want to spend a fortune to check few trouble codes. I am about to present you the cheap do it yourself route. Follow me:

Before you begin this tutorial

The VAG-COM KKL cable that i have used works only on certain cars. Please read and determine if your vehicle is covered.

  • 1994 and newer VW’s are covered.
  • 2005-2009 Golf/GTI/Jetta/Rabbit are not covered.
  • This interface DOESN’T work if your vehicle requires direct CAN connection.

If your vehicle is covered then please follow all the steps below.


Buy the VAG-C-OM usb cable. I bought mine on ebay. the seller was alpha-bid and i paid $20.38 (with shipping)
Here is the product description:
This cable is used to connect your laptop to the OBD2 computer interface on your car.


After you have plugged in your vag-com usb cable to your pc ( i am running windows xp) install the driver for the usb cable. In case the cable didn’t come with the driver.
You can download it here

Now since you installed the usb to serial driver, you need to know what the virtual com port is. To do that go to start–>my computer –>right click on my computer—>choose properties
—>click on Hardware—>device manager


The usb is recognized on my computer as virtual COM3.



Install VAG-COM 409.1 from Ross-tech:


Fire up the VAG-COM interface.


Connect the the OBD2 cable to your car OBD2 port and TURN ON THE IGNITION KEY.


Click on the option tab and select the COM port in my case it’s COM3 (STEP 2 from before).


Click Apply and Save, now you go back to home screen and then Click Auto Scan.


Make sure you deselect the Airbag check box. It will take a while before it scans for all modules. Here is a sample of what i got- 2 errors.


Click on copy results and open notepad and past it there ( Ctrl +p or Cmd +p on a mac). Save the file.

Let’s start with the engine error code. It says the intake air temp is too high…hmm it’s sensor G42.

I opened up the Bentley repair manual for vw Mk3, and i was able to find the location of the Intake Air temperature sensor (IAT). So let’s see what’s wrong with this sensor by checking:

  1. The sensor connection.
  2. The sensor output.

Sure enough the sensor connection was loose so that might be it. But while at it let’s check the sensor output. The sensor is nothing but a variable resistor that changes with temperature.
Here is a snap shot from the bentley manual showing the relationship between the sensor temperature and it’s resistance.
Ok now, let’s see what temperature is the sensor at. Using the infrared temp sensor the temperature was ~49 degree C ( semi-hot engine) . Looking at the graph above that corresponds to a resistance somwhere between 700~1000 ohms.
After checking the terminal resistance with the ohm meter the resistance was about 770 ohms. YES! the sensor is working!

Ok now while at it and since the sensor is working it’s good idea to clean the sensor connection. I used the CRC cleaner. It doesn’t leave any residues behind and it’s ideal for this job.
Ok I secured the sensor connection and Done! I went back to VAG-COM to reset the code. NO MORE CHECK ENGINE LIGHT. Wuhuuuuuuu!
Now to replace the ABS speed sensor follow my other DIY.

OK now let’s see how much you could have spent if you took it to a mechanic shop.
Using the mechanic scanner—–>$60~$80.

Most likely the mechanic will replace the sensor and will charge you 4~5 x for parts + repair= $100+$30 =$130.

You have just saved $190

Your cost ~$20

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