CEL with no codes?

[jetjock15]

I got a new tune i have been trying its a tow tune, tows great... i have towed the 40 ft fifthwheel over 400 miles with it through a few mountain passes, no issues... however twice now when just gentle driving around town at a stoplight or when just driving at constant speed the CEL came on.... two weeks ago was first time, i pulled the code it was a P0004 which i have deterimined is no big deal... today while going 5mph rolling down the street the CEL came on with no codes??? whats that mean.... Both times i just went back to stock program and as soon as it fired up the CEL was gone... ideals? thanks

Rob