Someone explain to me how...
My guess is because for a engine to burn cleaner and complete it needs to be at a certain temp. If you changing the operating temp it could make it run rich and it also changes the conditions it was federal regulated by. Thats just my guess in a nut shell I didnt know they were not legal though.
Aftermarket parts require an Executive Order. Even if the part is within emissions tolerances it must still be approved by CARB and issued an EO number. Applying and retrieving an EO number is very costly. That's why you see some aftermarket companies who make 50 state legal engines yet the parts used in said engine may not have an EO number all on their own. The company would rather have the EO number for the entire package than each individual part.
Aftermarket parts require an Executive Order. Even if the part is within emissions tolerances it must still be approved by CARB and issued an EO number. Applying and retrieving an EO number is very costly. That's why you see some aftermarket companies who make 50 state legal engines yet the parts used in said engine may not have an EO number all on their own. The company would rather have the EO number for the entire package than each individual part.
edit: personally the same 160t-stat(hypertech) has been on my vehicle for 8yrs w/no complaints
between that and my rad I've never seen my temps climb over 180 on the hottest bumper to bumper days 
