43 is a ESC issue, electronic spark control.
go here and follow this... i had a 43 on the monte i built and this let my find the broken wire....
http://www.chevythunder.com/maf_esc_module_code_43.htm
and yes i know it says 86-89, but it worked for me on my speed density setup. i just had to cross reference some wire colors for the different ecms. the ecm pinouts can also be found elsewhere on that site