The transmission will need adequate fluid pressure in order to shift.
The shifting issue can be due to having low or contaminated fluid, bad transmission oil pump or faulty shift solenoid.
Weak pump may not provide enough fluid pressure which can lead to the solenoid not working properly.
Take a look at this article, it covers the basics of automatic transmission issues:
Many times, with shifting issues the check engine light gets illuminated. Is the check engine light on? If so, getting is scanned for code(s) can help with the diagnostics.
I have attached diagrams for your reference.
Please let me know if you have any questions.
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Monday, October 25th, 2021 AT 11:02 PM