FYI:

My engine light kicked on after about 25 minutes of highway driving
on my Honda Accord with 155K miles.. The OBD2 error code is
Transmission related - P0740 (Faulty LockUp Control System). There
are no other symptoms - tranny shifts fine.

Honda mechanics said try to change the trans fluid a couple of times
and it might not come back on for quite some time. Although I change
fluids per the maintenance schedule, the old fluid came out pretty
dirty. There were some metal fragments on the plug magnet but nothing
more than in the past. After the first oil change, drove the car a
couple of days and then changed it again. The engine light hasn't
come back on.

You can find web sources that say P0740 is the kiss of death of a
tranny for 1998 through about 2000. I believe the Honda mechanic
told me it was indicative of the hydralic converter showing wear and
that it would eventually might require repair. If you work through the
service manual, there are various cheaper to repair problems but the
mechanic told me that each of those problems (e.g., replace celenoids
on top of tranny) will show symptoms (e.g., difficulty shifting).

I plan on changing the fluid more agressively - say every 20K instead
of the scheduled 30K.