Tuesday, 2 November 2010

Passat B5 Throttle Body & ECU Reset (Battery Replacement)

This is a very simple guide but often overlooked, especially when having a flat battery that needs replacing as thoughts are usually around the cost of the battery and having to extract the old one and dispose of it.  But this is the ideal opportunity to reset the ECU and throttle body which should optimise the cars running and clear any error codes.

Using a 10mm spanner remove the negative terminal from the battery and leave off for at least an hour, this will give the time for everything to reset internally.  If you are replacing the battery you will obviously need to remove the positive terminal and the battery holding plate located to the side of the positive terminal.  You will need a 10mm socket with a ratchet and extension bar for clearance.  Refit the terminals positive first so as not to throw a spark from the battery and possibly short a connection which could blow a fuse.

Battery dimensions

Firstly I tried resetting the throttle body using the following method:

  • Leave the drivers door open and turn the ignition to the first position but do not start the car
  • Wait for approximately 3 minutes
  • Turn the ignition off

Secondly I used two methods to reset the ECU which seemed to do the trick:

  • Turn the ignition to the first position but do not start the car
  • Press down on the accelerator pedal fully and hold down for approximately 20 seconds
  • Release the pedal and turn off the ignition
  • Turn the ignition to the first position and do not start the car for approximately 5 minutes
  • Start the car but do not rev the engine
  • Allow the engine to idle for approximately 4 minutes while the rev needle hunts up and down
  • You will hear the engine note drop and the rev needle settle at around 900rpm
  • Turn engine off
  • Restart the engine and go for a good 10 minute drive around the block

Thats it!  I had explored several ideas and recommendations for this and found that once I performed the above methods everything settled and the car seems to run smoother now as well.  This is a very simple fix but to my knowledge hasn't been documented with any certainty but I can confirm that this worked on my UK 1997 1.8 20v SE model.

After replacing a dead battery you may find that you will need to reset the auto function for the electric windows, a feature that closes all electric windows and sunroof automatically.  Refer to your owners manual if unsure.
  • Ensure all windows are fully closed (and sunroof if applicable)
  • Exit the car and lock it using the key
  • Unlock the car and lock again holding the key in the locked position until the alarm LED starts to blink
  • The function should now have reset and work again.

VW Transponder Key

If you have any ECU fault codes that will not clear then you may need to seek professional guidance.  Alternatively you can purchase your own diagnostic code reader:


  1. Hiya, I`m a self employed mechanic and changed a battery on a `98 1.8 Passat and found what you found. I read your instructions above, followed them exactly and the result is a very happy customer who has saved about £100 by not having to have it towed/transported/reset by Listers ( VW main agent ). Many thanks indeed, and the guy wants your email to thank you personally. All the best..

  2. Hi, many thanks for the feedback. Im glad that you found the information useful. I started writing these pages to try and help others out with simple guides. Dan.

  3. Hiya,

    Just wanted to say that i have had problems with my thermostat no registering anything on the dash, taking forever to warm up, yet the engine was physically hot! _ that was replaced and i have now lost about 10-15mpg. I am going to try this method and see if the ECU has learned to leave the valve open for longer i will post back! 10/10 for the advice on here, please keep it up for us passat owners!

  4. Hi great post! Many thanks for this stuff. I used it on my passat 98 basic 1.6 ahl and it worked fine.

  5. I have a 1999 Volkswagen Passat and I have replaced spark plugs and wires fuel pump ,coil packs ,mass air sencor, battery fuel filler,relays, fuses fuel regulator , and starter , alternator, camshaft position sensor, crank Shaft sencor and still want start will trun over but sounds like it's out of gas