The modification below is definitely not my invention, however I publish it just to give some kind of a confirmation that the mods id working correctly. You can actually find the same info in few places.
So, what the mod does?
If you try to use antenna which is not too good for a specific band, the ATU will get switched off automatically when the SWR is worst than 1:3. Icom did it for reasons. We are going to add 100kΩ resistor, which will let you tune antennas up to SWR 1:6. You can also add resistor 68…75kΩ which will let you tune antennas up to SWR 1:10 – it is a very bad antenna, and as the result of tuning such a bad antenna, the RF voltage inside of ATU will drastically increase, which may result with damaging the ATU unit. So if you decide to use lower value resistor, do not use the max RF power!!! Somebody found that adding resistor 100kΩ which will let you tune antennas up to SWR 100kΩ is safe even with 100W. Icom was overprotective with setting limit to SWR 1:3.
Anyway, the mod is not advanced, but you need to have good eyes and be good with soldering very small elements.
First step remove the bottom cover, then locate the place to be modified:
Remove the masking green paint with anything sharp:
The 100kΩ resistor has to be soldered to the cleaned place (ground) and into one of these two resistors on the side indicated. It does not matter which you will choose as they are connected together. And results are shown in the two pictures below: