Turn the ignition key until the digital odometer displays, repeat three times in succession and then back to "On" (On/Off, On/Off, On/Off, On). At the fourth "On" the odometer will be replaced with codes. If no codes are present then "Done" will appear.

How accurate is this? When all that stuff was going on with my Jeep I did that a couple times and it never displayed a code, only done.

if your check engine light is on then this will pull down the same codes as the OBDII scanner

What does it actually check? My oil pressure gauge is all messed up and it is not reading a code. My engine light is not on just my check gauge light.

as far as I know it will only pull down codes from your check engine light

for quick reference:

97 and up codes (displayed in the odometer):
http://www.stu-offroad.com/engine/scancodes.htm

92-96 (flash counts from the Check Engine Light):
http://www.jeep-xj.info/HowtoECUcodes.htm
http://www.jedi.com/obiwan/jeep/dtc.html

Correct it will only pull "check engine light" codes.