I have a non-S 3500 and a 4500 with a X-Bow magnetometer. The mag compass reading on the 4500 has been drifting off so I did a magnetometer realignment on Saturday. Worked great on two flights. Today I ran into the same problem - way off, about 15 degrees out of adjustable range. No changes in the presence of magnetic items in the cockpit/baggage area. Sounds like a bad magnetometer? If so are there any plug and play replacements?