code 20 is the eld, check ur alternator fuse under the dash. u may have a short somewhere triggerin the cars system voltage to drop, hence the cel.

as for the knock sensor, did it come on when u swap ur motor or just recently? i would first check the knock sensor first to c if its not faulty or a cut in the wire. replaced with a good known to c if the problem is still there, if so, then something is goin in ur motor. good luck.