I have a 2018 RDX with about 25K miles. My Maintenance Minder tells me I'm about to need service "A123". "1" is rotate tires, "2" is replace air filters. Makes sense. But "3" is "Replace transfer-case fluid and transmission fluid". There is also a footnote saying that if I abuse my car (mountainous areas at low speed, etc.), I need to change transmission fluid more frequently than the Maintenance Minder says: first time at 60K, then every 30K. Wait, what? I'm not at 60K yet! I'm not even at 30K! And in NYC there aren't many mountains!

So the question is: how do I interpret the Maintenance Minder numbers?

 

All Honda automatic transmissions have required regular fluid changes since they were introduced in the 1970s. The trans service the MID is referring to is a simple drain & fill, replacing appropriately 1/3 of the fluid. There is a drain plug & fill hole.
Do you have an AWD version with a transfer case? Then that fluid needs changing as well.

Now why did you get these codes at 25k? Well the MID just keeps track of how many oil changes there have been. I bet in NYC, the oil life gets down to 15% much sooner than a more typical suburban driver. The oil life takes many things into account, like how many starts, how many of them were cold starts, how long the engine ran before being shut down, how fast the trips were. Lots of stuff. Stuff that inner city driving might make oil life less. You don't have to be in the mountains to have severe service.

So, you probably could wait until the next oil change to do what was once called a 30k service. It just set the codes now because 4 oil changes have passed. It is based on a 7500 mile interval, what it was for this engine before oil life was measure with an algorithm.

 

So you are saying that "3" really means a "simple drain and fill", not a "flush". OK, that makes a little more sense. I am still confused by the wording of it. The manual gives the impression that I have to replace transmission fluid now (which sounds like a flush), but the footnote implies that such replacement should be done only at 60K and only if the car is driving in mountainous areas -- and later otherwise. This footnote just makes no sense, given that I'm asked to do "3" at 25-30K! I wonder if I'm the only one who finds this confusing.

One more question: do I ever need to do the full flush?

 

Yes there is a reason to "flush" the fluid, here is one of them:

http://www.urvi.net/forumfiles/SB/B17-017.PDF

Note that, on the last page when it gets to the actual flush procedure, it explains that one never actually flushes a Honda automatic.
To replace all fluid, you change it three times with driving in between. And more than around the block, it needs go through all the gears and the torque converter needs to lock up. So the dealer will simulate that. Do it yourself folks might just change the fluid three weekends in a row.

I do do understand your confusion with the manual. Mine describes features not available even in an Advanced model level. I figured out that it is a sloppy rewrite of the MDX manual.

So, about how many miles is the MID getting down to 15% for you?

 

Ah, so the full flush (the triple refill with driving in between) is only done if something is wrong. The MID reminders are for simple drain-refills.

I didn't pay close attention to how many miles pass between oil changes, but I think it is about 5K. Meaning that MID lights up at 3500 after the last one.