Check Engine light on 95 90CS

[EGOR46 at aol.com]

The check engine light comes on once my 95 90CS reaches normal operating 
temperature .  I could pull the codes on my 91 100 by using a volt meter,  is 
this also possible with the 95 90?  If not, do I have any options other than 
the dealer's computer?  I have access to a parts car so changing out sensors, 
etc. is an option.  Is there any one problem area that may be the most likely 
cause considering that the light only comes on once the engine is fully 
warmed up?  I appreciate your advice.