isx smoking when regen

[Rawze]

First thing might be to test the coolant system with a pressure test set to ensure it is not building pressure because of a head gasket or other issue.

Smoke during a regen, depending on what it is doing may or may not be normal. it depends on the circumstances and other factors. Could also be coolant intrusion as you have stated you are having coolant level issues.

Being a bit more specific may help like what kind of truck, what kind of smoke and how bad it becomes, when it started to do this, regen history, age/miles, etc. and any other related info would likely allow others to provide some better answers.