When I head up to the mountains the Check Engine light comes on at about 5,000 ft. It goes out on the way back down. The OBD II code is P0404. Is the altitude a factor or is there a problem with the EGR? I'm at about 7,000 ft. where my cabin is. My truck is an "07 6.0

 

Yes, you have an EGR problem. The 6.0 has a history of issues with it. It may just be a sticky EGR valve that needs cleaning, or it may be more serious.

 

I cleaned the valve and removed all the carbon from down in the well. I'm going up to my cabin north of Fort Apache this morning and we'll see if I get a check engine light. Later

 

The check engine light came on again at about 6,000 ft. It went out later in the day and never came on again. Why would altitude make the light come on for EGR?