The transmission is going to "limp-in" mode which is second gear,
because its computer thinks there is something wrong with it. Maybe the
clutches are worn, maybe there is a bad clutch seal, maybe bad
hydraulics, maybe a bad input or output sensor. The only right way is
to have the transmission scanned for fault codes, then a proper
diagnosis can be done. Otherwise you could be throwing irrelevant parts
at the problem.
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