Stock Evo X MR beside ETS V2 Regular Exhaust.
Went WOT and the Asc service require light popped up..
Weather is 20 degree lol.
I'm overboosting because of this cold temperature right? Cars run fine, until I Floor it at 3rd Gear at 3krpm.

 

if you have a lot of miles on your car you might want to take off the heatshield and look at the manifold. It might have a hairline crack. Happened to me and I couldn't figure out why I was overboosting

 

The ASC light coming on during boost is certainly a tell-tale of over-boosting (I believe it's factory set to > 30psi). Like AlexDRS94, you can check your manifold for a crack, but it just sounds like the cold weather is a factor for it. It depends on what you're tuned for.
If you're tuned for 27 tapering to 24 and the light is coming on, then cold weather would probably be a cause, but if your target boost is lower than that, then there could be other issues.


I purposely have a main 27psi map and a low-boost 23psi map for winter. Can't be too safe!

 

Care to explain how a crack (which lets pressure out) causes over boosting (more pressure going in)?