I just finished a 97 GSR swap in my 96 civic and my ECU is coming up with Code 20. I had this exact problem with stock DX motor, a few weeks before I did the swap. We were able to start the car fine, but I just took the car out today for a test drive. When I was driving down the street, when i would get on the gas the car felt like it was about to die, so I let off the gas and it would go back to idle. So i pulled over and tried to rev the car up and I had the same problem. When I would get on the gas, the car would start to die but I would just let go and it would return to idle. So I heard it may be the alternator so I do the test, where you leave the car running and disconnect the ground terminal off the battery, and if it dies that means the alternator may be shot. So i disconnected it and the car died. So I reconnected the terminal and now the car would crank but just die right after you stop turning the key. If you keep cranking the ignition, it would crank but then just die out. I also noticed that the speedometer wasnt working but the tach was on my SI cluster, if that has any relation to my problem. The weird thing is I thought of replacing the alternator, but I had this same code on my old motor as well, so I really dont think both alternators could have been shot, maybe something is shorting it out? But can anybody help me out, because I am clueless right now.

 

check your grounds they might be corroded

 

had the same problem swap a 2000 gsr motor to my 96 dx hb with a 97 ecu it was my 7.5a fuse unders the driver side dash i think #18 or 15 one of them. Fixed the problem so far but still having a code 20