engine codes

97 xj 4.0l 5 speed
check engine light is on and i want to try to retrieve any engine code(s).
i was told i could do this by an on/off method with my ingion key.
how do i do this? how will i read it?
any help would be appericated

Turn the key to on,off,on off on then watck the "checkengine "light flash count the flashes. Example--flash-flash pause flash flash flash pause -would be a 23 code,it will do it 2 times and then move to the next sequence. A 5-5 will be the end.
Wayne

thanks wayne!
actally on my xj it flashed the code where the odometer is
the codes were 12 and 21
Does any one know what the are, or what they mean ??
thanks for the help

12 is start of sequence (normal).

21 is oxygen sensor or its circuit.

Here's a useful page for codes:

http://www.allpar.com/fix/codes.html