p1273 wtf is that? engine light after motor swap any ideas welcomed

did a motor swap vq40de direct swap put new upstream cat on driver side as well as o2 sensor put up/downstream cat on passenger side with new upstream o2 sensor. after the swap got a p0420 code bank 1 cleared after i installed those new o2 sensors (^^above) few days later get a weird code p1273 cylinder no.3 high to low side open(bank1)..... literally lost called local mechanic said its a exhaust leak or a vacuum where its sucking to much air. have no exhausts leak that i noticed all head bolts are tight to spec as well


my idea since i only replaced upstream o2 sensors could they be conflicting with each other? went with bosch o2 sensors while the downstreams are still the originals

if anyone has a fail proof way to test for exhaust leaks other then looking for black or putting your hand around the connection points that would be appreciated as well