Surging TBI

I decided not to go the route of getting a new engine but with doing so i began to search the engine for the knocking issue and in doing so i messed up my Idle Air-Control. So I went and got a new one and installed it fine. but the issue now is the engine light is on, so i checked it and im getting a code 42 which says "The EST signal did not change when the ECM applied bypass voltage to the ignition module." Which no one at the local auto-parts store seems to know what it means or what a EST is. Any idea on what can possibly be causing the Code 42 to be present?

Now a second issue arose, the TBI is surging at about 1500 rpms and goes away at about 1800-2000rpms. Above or below 1500rpms it runs ok. has this happend to any one before? if so whats causing it?

once again guys thanks for any info possible.