transmission shifting problems on 2001 Nissan Quest

a little over a year ago my service engine light came on and since the van was shifting hard I took it to amco for a diagnostic. the codes that came up were torque converter and engine knock sensor.I ended up buying a used transmission and things seemed ok but only for 3months. the van shifts hard once again and now it never shifts into any gear above 2nd. So my question is- should the focus be on the sensors at this point or did I just end up with another possibly bad transmission. The mechanic that installed the replacement tranny never did any of the "process of elimination" testing on the sensors that were listed in the diag codes. He just went straight for the transmission itself. Is it possible that the problem could be a sensor?

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1 answer
The knock sensor at worst would retard the ignition timing a few degrees wouldn't cause the transmission problems start by checking the transmission fluid level and condition.