Here is a pretty good explaination from ITP's website. My guess is you probobly have a map fooler of some kind, or your gauge isn't working.
The computer in your 7.3L Powerstroke Diesel equipped vehicle will turn on the "Service Engine Soon" light at about 24psi of boost (some calibrations may come on at lower pressures)...a pressure easily attained by many of todays high performance chips, tuners and modules. At the same time, the computer will reduce fueling (power) in an effort to bring the boost pressure down. This reduced power mode will remain in effect until the PCM sees enough continuous operation without the fault (usually a few minutes of driving), the ignition is cylced or the code that tripped the SES light is cleared electronically.