Please help.  Pulled code 42 from the OBD I ECM on 95 4WD Suburban K1500.  I was looking for code(s) that might provide insight into a trans that recently started slipping (flaring) between 1-2 and 2-3 upshifts.  What recomendations to isolate the root cause (edit- add) of code 42?  I'm a former auto technician with a degree in electrical engineering.  A little direction would be helpful.  All I have is a Chilton manual that does have some schematics.  Thanks for your time.

Code 42 relates to the Electronic Spark Timing (EST) system.  The computer puts out a square wave timing signal to the ignition module..  It also puts out a 5v 'Bypass' signal.  The bypass voltage causes an electronic switch in the module to click over to EST.  Without the 5v it's in the 'Bypass' position and runs off internal base timing.  In bypass the EST signal is pulled low by the module and in the EST mode, it is not.  

The following picture illustrates.  Top is bypass and bottom is EST.

http://www.autonerdz.com/images/sm1.gif

Something as simple as setting the ignition timing will set the code 42 because the bypass is unplugged to force base timing for adjustment.

Otherwise, the switch in the module was not following command, wiring , or computer problems.  I suggest just clearing the code and see if it comes back.  Probably not related to your 'flare'

If you need to find a qualified technician in your area with good access to information resources you may use this tool:

http://www.iatn.net/dir/search.asp

Thanks Tom.  That helps me understand what's going on.  It also prompts my aging memory.  I did check the timing a bit ago and did not reset the ECM.  Thanks for your help.