May be a silly question but do you guys keep these on blue at ALL times?

In the Pic blue light on means no fresh air. That setting is meant for brief period, not full time use.

 

Depending on the truck, that blue light indicates the statues of the a/c system. Blue light illuminates, indicating the system is ‘on’, no illumination the system is ‘off’. That ‘max’ button you see, press that while the a/c system is ‘on’ and that button may light up, that will ‘recirculate’ the air inside the cab, thus maximizing the cooling of the air.

 

Leaving the recirculation/blue light on all the time is like wearing a plastic bag over your head. You are re-breathing your exhaled breath. If you are smelling exhaust smells in your truck you have an exhaust leak that needs to be fixed. I would only turn on the recirc/blue light like on rainy days and your windshield fogs up after you get back in the truck. Turn on recirc & run defrost.

 

You've got that backwards. When the blue light is off, fresh air is being drawn in from outside, and when the blue light is on, the air inside the cab is being recirculated.

I keep mine on recirculate (light on). But I run in a hot, very humid area so my A/C needs all the help it can get.