Just had this bug for the first time (SH4 1.5 US campaign)..... read through this thread and tried hitting the CO2 icon, but that didn't work for me
The Co2 guage kept increasing as I travelled on the surface.
It only cleared for me when I saved the game and then loaded the save (no need to exit the game or quit to main menu). Now everytime I load one of the saved games where I know the bug was present, I see the Co2 icon when the game loads, but it clears instantly.
As long as that works it's not too much of a problem, but if things get worse I may need to change my gameplay settings ............ I'm got going to suffocate on a patrol becuase of a bug !