Oh sorry , yes all 4 match....Summer setup are 20"s staggered but winter setup all matching 18's....oh but wait , would 2 brand new tires with 2 more used tire do anything?
I put the 2 new ones in the back and other 2 with about 60% left on them in front.....

No, treadwear shouldn't matter too much, but maybe with winter tires usually having deeper tread, that has a small chance of causing an issue. Even if you were almost to the min. tread depth, that's going to be only about a 2% difference in overall diameter between the worn and new tires, so not significant enough to cause a problem.

Since all 4 are the same size, now maybe we should know what size are the 18's? You should have 255/55/18 to get a correct speed reading. Maybe DSC is worried that your wheel speed vs. transmission speed is different than before? Normally, this would cause the flat tire monitor to come on, but all 4 wheels travelling at the same RPM would prevent that from happening.

255/55/18 is what I have.....

If two of the 18" winter tires are used/older....and the other two 18" winter tires are new...and every time you mount them on the vehicle the DSC/4x4 light illuminates...but if you mount your summer 20" wheel/tires...the light goes off...isn't there a big ELEPHANT standing in the middle of the room in that there's something off with the 18" winter wheel/tire set-up?

If the overall diameter/circumference of the wheel/tires is off by 3% (+/-)...it can create an issue. And perhaps a simple check to see how far off the overall circumference of the wheel/tires on each axle are...may reveal what the issue is.

Are the tires on the front & rear axles the same brand and size (just double checking)? Or do you have dedicated winter tires on one axle and all season tires on the other axle (even though the both may be 255/55/18)? And which axle has the "old" tires...and again, what is the tread depth?

What is the tread depth of the old 255/55/18 winter tires vs the tread depth of the new 255/55/18 winter tires? If you don't have a tread depth gauge...stop by any tire vendor and they can measure this for you...and probably for free...especially if they think you may be buying a pair of tires from them. :)

If you don't want to do this simple task...then as suggested in other replies...hook the vehicle up to diagnostic equipment so that you can communicate with the vehicle to find out what faults the DSC/4x4 system has registered.

Check the air sensor levers. Sometime they will flip over when you let the jack down. The arms should be toward the outside on that side.

Using the first recommended pair of snow tires on tirerack for a 4.8is on 18" wheels, the tread depth is 13/32" new. Assuming same model tire used all the wear to the wear bars, 2/32", would give a total diameter change of 22/32" (2 x 11/32). The new tire has a diameter of 29", subtract the worn tread depth, then divided by new diameter, 97.6%, or a 2.4% difference. Since rolling/loaded diameter is different than unladen, its very possible to go over 3%.
What about tire pressures? Snow tires have softer sidewalls and seem to be more affected by tire pressure differences. If using the recommended pressures on the door, I could see just that causing a rolling diameter difference, compounded with tread wear difference...I run my snow tires same pressure all around, haven't noticed an issue, same deal 20" summer tires, switching to 255/55/18 snows.

wouldn't this throw a self-leveling warning?

Normally yes--I missed the part where you drove 10-20km--not the leveling arms.

I'll double check everything you guys brought up...but to answer some questions:
-all tires are the same brand/model and size ( scorpions).
-both new tires are in the back and older ones front
-I'll check the tread depth.....

:thumbup:

Just get a tape measure and measure the circumference of all the wheels