Oxygen sensor problem

I just changed the engine on my 97 Taurus and now I'm getting two Oxygen Sensor trouble codes, P0136 and P0155. One is for the pre-catalytic converter on one bank and one is for the post-catalytic converter on the
other bank. Before I put the new engine in, all the Oxygen Sensors were fine and working. I'm skeptical because what are the odds that both would go simultaneously. I'm thinking that maybe something isn't right with the electrical harness that came with the new engine in which all the sensors plug into. If anyone could let me know what is likely to be the problem I'd be most grateful because something tells me that if I spend $150 on two new sensors that the problem won't just go away.
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RawPhenomenon via CarKB.com wrote:

You could be right about replacing the sensors. If they tripped the codes they're probably just doing their job. You need the pinpoint tests from the factory service manuals to troubleshoot the 02 sensors, sensor circuits, what mechanical symptoms to look for, etc.
Have you also considered something about the engine characteristics that would cause the O2 sensors to trip the codes? Burning lean/rich, not closing the feedback loop, etc.? Any other codes?
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