When trans warms up will not shift down. Stays in third gear.

Try changing the transmission fluid with Mobil 1 Synthetic Multi-Vehicle ATF (3 quarts), after it reaches normal operating temperature. It meets Honda's Z-1 specs. If the problem remains, you should take it to Honda for diagnosis. My bet, however, is this will work. Since this procedure will only change half of the transmission fluid, I would do it again soon. Easy job.