Cluster problems ...........1996

I installed a new cluster ( junk yard cluster i should say ) I installed the cluster and the car will start for about 3 sec. then dies . So i unhooked it and hooked up the old cluster and the car started right up. So with the car running i reinstalled the new cluster and the cluster works fine....... Today i had a check engine lite come on along with the ETS lite....I took the car to my shop and scanned the codes (P420) so i cleaded the code and the car would not start again. i install the old cluster again and it restarted , So left the car running and reinstalled the new cluster an the codes are gone. But why would this thing start after i installed the newer cluster ????? :crying:

UniX