Mike, thanks for all the work you're doing.
I do need to note that there is a huge difference between the 5at in the Gen 7 vs the 4at in the gen 6. Both V6s mind you.
The gen 7 has torque reduction during shifts, it retards timing and closes the throttle to reduce engine power during shifts. The gen 6 does not.
Along with a bunch of differences internally, trying to compare the failures that the 4at sees to the 5at is like apples and oranges. Yes, they use some very similar parts, like the pressure switches. I've only ever seen pressure switch issues with gen 6, none yet on gen 7s.
Yours may be a first (pressure switch issues), but changing the fluid as you did, and the filter probably raised up internal pressure. The PCM will relearn the new fluid over time. They really make no mention of it, but in my experience it does. There's no explanation as to why the shift logic, feel and gear engagement change a while afterward. I'll have to look at scan data on a gen 7 5at to see if it actually is in 3rd when you go from P to D.
I know the 4at goes directly to 1st. I've had several that have had no 1st gear, and putting it in D2 will allow you to move them around.
You are correct that cars that see much city use, do fail much faster. The 4at loses 2nd gear, and the 5at usually loses 3rd. These are the most common gears that are pulling the most load in stop go conditions. Considering that the fluid is superheated by the torque converter and there's so little heat rejection capability in the cooler system, it only serves to cook them faster.
There is one thing I've noticed in one of your other posts, that I can't wrap my head around. In a constant mesh trans such as this, it CAN'T have two gears engaged at one time. In any manual trans, if you do so, the trans LOCKS solid and won't turn. I can't see the unit ever having two clutch packs for different gears ever engaged at once.
I think the reason for the flaring, is that over time, with clutch pack wear, and the ever present valve body wear in these units, they can't immediately release and engage the next pack fast enough and thus the slight or huge flare depending on how long it takes pressure to build or clutch clearance to be closed up.
The first Honda autos used to shift extremely harsh from gear to gear due to their need to have this immediate release/apply situation.
I remember reading that these units also have a way to accumulate pressure behind the clutch apply valves in such a way that allows them to vent pressure out of an applied clutch and then apply the next. Later, the adding of the shift solenoids softens this application, so that it doesn't do the harsh jump like the old ones. PWM of the solenoid allows this provided the unit can build up the pressure in the first place. The line pressure is also PWM up and down via the linear solenoid as needed during shifts.
If you ever notice on a failing unit that flares badly, if you run it at WOT they ALL (in my experience) will shift perfectly as long as it was able to engage the gear that flares at lower throttle settings. If you can manually raise line pressure via the linear solenoid, it'll shift HARSH at light throttle, but the flare usually goes away, and the WOT shifts feel fine.
I've had luck with only a couple that have minor to moderate flaring, getting them good and hot, dumping the fluid 3 or 4x, then disconnecting the battery, hooking it back up and immediately getting on the road and running the trans at WOT numerous times.
I just worked on a 2003 Pilot today, it's got the infamous 5at with the jet kit. It's got 226k on its ORIGINAL trans, that's been serviced every 30 to 50k miles. It still shifts perfectly, although it does whine very loudly at constant speeds.
Sapphire blue '04 V6 6 speed w/ full HFP, comptech ss, momo shadow, TL-S 27.2h/20 bars, intake spacer, retro chrome MH1 HIR2 highs, hella DE fogs, EBC Ultimax, subwoofer/amp, infinity rears, HD Radio, aux adapter, navi conversion.
Vermilion 1999 SVT F150 Lightning, Sylvania/Visteon Xenarc HID headlamps, '01 L turn/marker lamps, PIAA fogs, '01 up Bilstein shocks, all Pioneer speakers, Clarion subwoofer. All Redline fluids.
Silverstone 2003 Honda S2000, bone stock