Transmission shifting

When an automatic transmission will not shift out of first gear, it might caused by a low fluid level. This can cause many shift problems and should be the first thing you check. If fluid is at the proper level and clean, check linkage, especially if the vehicle is older or high on miles. Linkage can wear out and cables can stretch. Next, the problem could be failed shift solenoids or electrical. Might be as simple as a bad speed sensor or a PCM problem. Next, and this might seem somewhat remote, but if you lose a catalytic converter heat shield, that could cause overheating and transmission internal wear.

If you can shift manually but the automatic side is not functioning it could be a couple of things, gear selector switch not sending the correct signals to the computer, computer lost shift control programming due to a power loss. A live data scan can read this information so that is what you should do before throwing parts at it.