It's not a passive unit. It's automatic and doesn't require module changes. The company said with the cold weather I may need to reposition it to be away from the cold air. It doesn't activate until it reaches a certain temp. So if it's in the stream of cold air it can affect it.If passive muzzler, that's what you get for getting a cheaper muzzler. You have to adjust it as the weather changes.
In this case since it's reading the cold air temperature, it's likely decreasing resistance since not as much is needed when the temperature is lower, so the ECO light is able to come on.I was a little confused when I first read this thread. The passive muzzlers (MaxMuzzler or VCMuzzlerII) tend to show the ECO light on when it's HOT, not cold, outside, since the resistance isn't strong enough in the heat. In the cold, the passive resistors tend to trip a check engine light for coolant temperature sensor if the resistance is too high.