do I ever have to get a Trans Flush?
I have a 2009 Sonata. If I change my transmission fluid every 30,000 miles on 2009 Hyundai Sonata
30,000 mile trans oil changes are good, maybe a little more frequent after 150k if it is the original trans at that stage. I personally don't like "flushing" machines, have seen them dislodge debris causing problems rather than helping matters. Dropping the trans pan and changing the filter were applicable is best.
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In most cases, if you change your transmission fluid every 30K then you should not need a transmission fluid flush. However, if the fluid were to become discolored or brunt smelling then a flush would be in order.