The "diagnostic display" mode is entered with the following Procedure: Page 8-500

1) Turn on the ignition but don't start the engine.
2) Press the "reset" button to turn off any warning messages
3) Press and hold "options" and
4) While holding "options", press "fuel" four times within a 10-Second period.

Initially, the on-board diagnostics go into the "automatic" mode, which shows each module's DTCs in a
pre-set sequence: (Page No. are for the GM Service Manual)
10 PCNI Powertrain Control Module page 6-357 - 6-361
28 TCS Traction Control system ABS on page 5-86
?? RTD Real Time damping page 3-136
40 BCM Body Control Module page 8-405 LTD Page 8-727
60 [PC instrument Pane! Cluster page 8-508
80 radio page 8-213
99 FIVAC Heater Vent-Air Conditioning page 1-118
A0 LDCM Left Door Control module page 8-904 to 8-951
Al RDCM Right Door Control Module page 8-904 to 8-951
AC SCM Seat Control module page 8-1064-8-1082
B0 RFA Remote Function Actuation page 8-676

For each module, all DTCs will be displayed. If none are present in a module, you will see "no more codes" on the ICP display.
There are two kinds of DTCs, "Current" and "History," designated with a letter suffix, "C" or "H". A current code indicates that the malfunction is present in the system whose module is displaying data. A history code indicates a problem existed in that module sometime in the last 40 or 50 ignition cycles. When not accompanied by a current code of the same number, it is possible it's evidence of a previous problem. Now solved, that was not removed by clearing codes. More likely is that a history code indicates an intermittent malfunction.

"Intermittent" are the most challenging DTCs. An intermittent may have happened only once, may have happened more than once but is inconsistent in its appearance or may be happening on a regular basis but not at the time the IPC is displaying codes. History codes can also be caused by a current malfunction in a system that is not Operating at the time DTCs are displayed. An example is the rear window defogger, which doesn't operate until the BCM detects engine rpm. For history codes set by a system that does not operate with the key on and engine off, a special diagnostic tool called a "scan tester is necessary to properly diagnose the malfunction.

Once the IPC has displayed all 11 modules, the system goes into the manual mode, which allows selection of each module, using combinations of DIC buttons. The manual mode can also be entered at any time during the automatic sequence by pressing any button except "E/M". Once the IPC displays "manual diagnostics", you may select a particular module by pressing the options button to go forward or the "trip" button to go back. Once a system is selected and a DTC is displayed, if more than one are present; press "gages" To move forward or "fuel" to go back. To exit the diagnostic mode at any time, press E/M". If you want to erase or "clear" codes, press "reset". Clearing a code does not repair a problem. You are simply erasing the evidence of it in the module's memory.