C5 Check Engine Light Code Reading Procedure

The "diagnostic display" mode is entered with the following procedure;1) Turn on the ignition but don't start the engine.2) Press the "reset" button to turn off any warning messages3) Press and hold "options" , press "fuel" four times within a 10-second period.When first engaged the on-board diagnostics go into an "automatic" mode and cycle through all the modules and their codes automatically. When the screen displays "manual diagnostics," select the desired module by pressing the "options" button to go forward or the "trip" button to go back. Press "gages" to go forward or "fuel" to go back for each code under a module.To exit the diagnostic mode at any time, press "E/M". If you want to erase or "clear" codes, press, "reset."The computer will show two kinds of codes, "current" and "history," designated "C" or "H". A current code indicates a malfunction is present in the system whose module is displaying data. A history code indicates a past problem. You can ignore "U" codes