Just in the nature of general troubleshooting... have you checked all the connections to the autopilot... ensured that nothing was knocked loose as wires were passed about. Especially check that all connectors are well seated and in good shape. (sometimes connectors appear to be in good condition, but a wire has broken inside when the thing was bent to some extreme angle)

Then see if there is a calibration procedure for the Autopilot and see if you can enable that calibration procedure. It sounds as though the AP is physically working, that only the fluxgate unit is either giving bad data or out of cal.

Do you know where the fluxgate is physically located? There isn't any chance that some speaker (magnet) got repositioned in the ICOM install?

Just for grins... since the ICOM is hardwired to the battery... disconnect those connections before you try the AP again.

BTW your assumption that the AP was working fine may ultimately prove to be false... yup, stuff happens. Good luck.