Go into the Service Mode and check the following:

1. GPS status. You should see "Position known"

2. GPS S/N signal strength figures. You need 12-16 on at least 3 satellites. 20 is really good, under 10 is weak

3. PDOP figures. PDOP of 1.0 is super accurate positioning, less than say 3 is normal. 5 to 10 is inaccurate, above 10 is really bad.

4. Gyro - ist it at 2500 +/- 100mV when car is stable, does it go down when turning left, up when turning right?

5. Wheel pulse reading - is it 0 when still, increase with speed, and show negative when going backwards?

6. Direction indicator: should be backwards when in reverse gear, forwards all other gears.

Report your findings.
Also, state your nav software version (top right of Settings screen)

With the right software tools, you can also retrieve the diagnostic information from the nav. However the Service Mode tells you 90% of all data.

To enter service mode:
Go into Settings
Press and hold MENU and wait approx 10-15 seconds
Nav will switch to Service Mode.