There are a few issues that cause this. First if the vacuum module is bad or no vacuum is getting to it the trans. will not shift on its own. Second is the trans is extremely dirty inside and need service. Some transmissions are electronically controlled and if the relay or the internal electronics are bad it will not shift automatically. Good news is if the trans will shift manually the issue is minor. Check your vacuums first, Good luck
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There are a few issues that cause this. First if the vacuum module is bad or no vacuum is getting to it the trans. will not shift on its own. Second is the trans is extremely dirty inside and need service. Some transmissions are electronically controlled and if the relay or the internal electronics are bad it will not shift automatically. Good news is if the trans will shift manually the issue is minor. Check your vacuums first, Good luck