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International Automotive Technicians Network
Re: 90 Chevy Truck, code 42
Posted to Technical Discussion Forum on 1/20/2005

Hi Emil,

Have you verified the EST (white) and Bypass Circuits (tan/blk)between the ECM and ICM? Do you see a .5 square wave on EST that kinda mirrors Reference (Pur/Wht)when it's running? You could put a "hot" testlight on Bypass just to see if it opens up ... Login to read more.