I am lost in the weeds here. I have a 2000 GXE with 142,000 meticulously maintained miles. This car performs flawlessly except for a harsh 'clunk' when slowing down to a stop and trans downshifts to 1st. The only time it will not do this is during aggressive braking to a stop - no clunk. A mild clunk sometimes occurs when accelerating moderately, then back off throttle just as 2nd to 3rd shift occurs, and clunk. This is secondary to the downshift issue which occurs almost always. I have verified - all mounts good, drive axles good, all running gear good. TPS perfect and no trans codes. I succumbed to the rhetoric and replaced the solenoids - no change. This has stumped me and all my expert wrench head experience.

Welcome aboard Jim!

Did you run the TCM self-diagnosis in the FSM? Beyond that, I'd look at an overfill condition or clogged filter (which I'm sure you'd have addressed when the pan was dropped).

OBD test results are OD light steady on = PNP switch, OD switch, TCM. I'm leaning PNP switch - it's mechanical, exposed to heat + environment, and more prone to failure. OD switch maybe. TCM - 31036-3Y161 is NLA. (only reman) The one thing I don't see much discussion about is fluid types. I am using Castrol Transmax Import. I've often wondered if this issue could be due to something as basic as fluid brand, additive package, etc. I have done many drain & refills. I suppose I could replace with Nissan fluid but it would take years of D&R's to purge fluids completely. I may just try to ignore this annoyance and drive it.

Replaced PNP switch. Still clunks. I called several dealers and spoke with their service departments. The kids there did not seem to have any clue really about anything. I did find a dealer with an old salt in the parts dept who said he recalled how these transmissions were complicated, expensive, reliable, BUT very finicky about fluid. Based on everything I have done so far, and the condition of the transmission, my last option is: I am going to buy Nissan Matic-D and start D&R's.

Whoa wait - I missed that in your first post.

YES. Don't use "compatible" fluid, use the Nissan fluid. That is a real thing (found it out the hard way on a buddy's Titan).

BTW, it won't take "years" of D&Rs - Drop the pan, let it drain overnight. You'll get 95% of the fluid. If you're REALLY persnickety, pull one of the ATF cooler lines from the trans and purge it with air. Then you've drained 100%.

Sorry for the late update. Clunk in downshift solved by replacing the TPS. The Trottle Position Sensor incorporates a throttle closed switch that gives input to the transmission control module when to disengage the torqe converter while decelerating and downshifting from 2nd to 1st. Even though I checked, tested, and adjusted mine per specs several times, I decided to replace the TPS anyway. The clunking on downshifting is gone. This was most likely the culprit all along. If you are having strange unexplainable down shifting issues with your gen 5 Maxima, try replcing the TPS.