Being new to this forum I’m not sure if this has been mentioned before, but I had exactly the same problem with my 1999 Mk 4. The problem first manifested itself about two months ago when I was driving to an obscure address in Coventry. I didn’t switch the sat nav on until I stopped for petrol on the A45 just outside Coventry, but when I did it was telling me that I was still outside my house just outside Oxford. I managed to zero it back in using the calibrate function, but it wandered off course again within a couple of miles. Bit of a nuisance really as I was relying on it to find my destination, but luckily I had an iphone with me so I was able to use the sat nav function in that instead. Over the next month I tried zeroing it in to my current location several times, but each time it would only last for a few miles before wandering off track again. I assume this was because the car was using only the “dead reckoning” part of the system where it works only by knowing the direction of travel and by counting the turns of the wheels, hence the problem if you change the tyres. All this time the GPS logo on the screen was not lit. However whilst scouring this forum, and the Club Lexus one in America, I chanced on the method of accessing the so called “secret menu” (Climate and Audio held down together and turn the lights on and off twice, with the ignition in the auxiliary position) When I did this a screen that I had never seen before appeared and then the car went through as series of checks including one for the sat nav which it declared to be OK. If you then select sat nav on this menu another screen appears and one of the options on it is GPS check. On selecting that, a number of vertical columns of numbers appeared, and as I sat watching them they were flickering and changing. I didn’t really know what I was looking at, but it did look like it might well have been satellite reception. You then have to press the map button to exit this menu, and when the map screen appeared it now showed the car as being in the correct position, which it wasn’t when I started, although the GPS icon was still not lit. However the next time I used the car, the GPS icon reappeared and the sat nav was working correctly, and it has continued to do so ever since. Not sure if this will cure Matthew’s problem but it certainly cured mine, at least for now.