code 43 est failure

Ok guys/gals
I need some help!!
I have a 91 k2500 4x4 with 454 engine ,
I am getting a code 43 est failure , I have replaced the knock sensor, checked the wiring, even ran a new wire, found the distributor bushings were bad so I replaced the distributor and ignition module,
I can not get rid off this code, as soon as the engine runs for a few seconds the service engine light comes on , i have also checked the base timing , at 4 deg ,
oh yeah I also replaced the ecm as i learned the esc module is built into the ecm on this truck ,
anyone have any suggestions ?
my next step is to check timing chain , I don't believe it is bad but
please reply


thanks
Michwrenchman