Originally Posted by KiwiJochen

When the car looses battery power, you can loose the nav calibration. To recalibrate the nav (with a fully charged battery of course) do the following:

1. Ensure satellite almanac is loaded. Check the almanac is loaded in the Service Menu. If you see Almanac=no then part the car outside, with boot closed, and no overhead obstructions so it can collect satellite data. Turn ign key to Pos1, switch on nav, and wait 15min. Confirm alamanc is loaded.
To access Service Mode, from the main ment go into Settings (or Set, depends on loaded language), then press and hold MENU until service menu appears. Enter the GPS Status screen and observe. Take a moment to confirm good satellite reception (S/N around 11-15 for at least 2 satellites means antenna is good)

2. Cailbrate gyro compass. Gyro is self-calibrated every time you lock and leave the car. Vehicle must be still as the calibration run is done. So close and lock the car, and then wait 10min without bumping the vehicle.

3. Calibrate speed pulses. These self calibrate when the car is driving by comparing actual speed with GPS speed. To speed up calibration, drive the car on a highway at max highway speed, constantly, for about 10min. Note that Mk4 navs can take a few days for the calibration to be spot on.

3. Calibrate map with reality. Every time you turn from one street into another, the nav realigns the map with your cars position. So drive through a suburb with lots of 90 degree street corners. Every time you turn a street corner, the nav will adjust the map. Do a few in each direction.

4. Be patient. The Mk4 nav can take up to a week of driving before the nav is perfectly calibrated to your area.

5. And sometimes the map is too far out. If the map data is too inaccurate (this does happen!) then the nav may choose the wrong road. Nothing you can do about this apart from choosing a newer, or older map.