10-30-2003 -- New information about possible link from O2 sensor/rich air-fuel mixture, and harsh shift condition, with due credit to original posters:
The van suffered a very hard shift condition when shifting from first to second, & to a lesser extent when shifting from second to third. It was intermittent at first, it would usually shift normally when cold & then worsen while driving. The transmission would shift & perform flawlessly when not doing it. When it did malfunction, upon the next re-start, the O/D-OFF light would flash a code of 16 long blinks follow by 5 short blinks which I was unable to cross-reference with the shop manual. A short while later, the CEL remained on constantly. The code obtained from that was 1205 which is "Line pressure solenoid and/or circuit fault". It was time for a fluid change so from prior posts which indicated that had resolved harsh shifts, I changed the fluid & filter which helped some but didn't solve it & the CEL remained on. Since it usually only happened when warmed up, I checked the resistance of the temperature sensor while I had the pan off & it checked out within spec.
Off to the shop it went. They quickly confirmed the Line pressure solenoid was faulty along with the control body solenoid pack & replaced them which helped some but still didn't fix the problem. After relacing them, now it gave a trouble code which they were able to determine was the "Dropping Resistor". Nothing in the shop manual gives any information about replacing the dropping resistor or how it fits into the diagnostic scheme, so I surfed the web & ran across references that bypassing or shorting it with a toggle switch is one method used in earlier Datsun & Nissan drag racers to intentionally get hard shifts, coincidently the same symptoms I was experiencing, so I had the shop replace it. The Dropping Resistor is located under the air intake housing behind & below the battery, along the inner left fender well. It isn't cheap, $71.75, and had to be special ordered. Unfortunately the new dropping resistor didn't fix the problem either.
Next step was to have the shop sub it to Nissan. Their diagnostics said it was either a break or short in one of the wiring harnesses, but they didn't have a clue where to start looking for it. They finally swapped the transmission controller with a good used one & that fixed the problem. Nissan only sells transmission controllers on a rebuilt/exchange basis, and they're not cheap, $400+. Used ones average about $125 at a salvage yard. Check http://www.car-part.com to do a search for used ones. You can also send your controller to http://www.fulllogic.com to be rebuilt.
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