Why doesn't a 1991 Mazda 626 automatic upshift into over drive?

A 1991 Mazda 626 may not upshift into overdrive due to several potential issues, including low Transmission Fluid levels, a malfunctioning transmission control module, or a failed shift solenoid. Additionally, a faulty throttle position sensor or a worn-out governor can prevent proper shifting. It's also possible that the vehicle remains in lower gears due to an engine problem or if the transmission is in "limp" mode to protect itself from further damage. A thorough diagnostic check is recommended to pinpoint the exact cause.