2010 Mazda3 S hatchback with automatic transmission and about 90k miles. The issue is intermittent, but seems to happen most often within the first 15 minutes of driving the car.
It simply won't shift up from 3rd without me putting it into manual mode and doing it myself, after which it seems to have no trouble. When it does work, it seems to be shifting at higher RPMs than it used to, but that could just me being paranoid and making it up. No warning lights in the cluster.
Thoughts? Shifter solenoid or something like that? Speed sensor?