Why won't 2002 Pontiac Bonneville shift into overdrive?

A 2002 Pontiac Bonneville may not shift into overdrive due to several potential issues, including low Transmission Fluid levels, a malfunctioning transmission control module, or a faulty throttle position sensor. Additionally, problems with the transmission itself, such as worn clutches or internal damage, could prevent overdrive engagement. It's advisable to have a professional mechanic diagnose the issue to pinpoint the exact cause and recommend appropriate repairs.