I had an open ground on my heated O2 sensor harness. A buddy loaned me his Matco Scanner. It saved me with all the tests you can do on it, but it still took some sleuthing. Mine was going into limp mode after the first minute of running, because the ground was open. I fixed that and it ran normally.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)