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'09 SF 3.3L with120K miles. Believe it's the 5 speed automatic. Two weeks ago shifting into reverse gave a small shudder, then everything OK. Now reverse slips and goes very slowly. Forward gears seem just fine, so far. Fluid level is at correct level on the dipstick. I drained and filled the AT fluid 5500 miles ago and all was well. Filled it with Valvoline MaxLife synthetic. Will another drain and fill be worth it? Or should I take it in to the shop? Is this the beginning of the end for this transmission? Thanks.
 

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Thought I would check for any trouble codes, though the check engine light is not on. It turns out there is a P0734 code, "Gear 4 incorrect ratio." Is this more trouble, or does it somehow relate to the slipping reverse gear? Thanks.
 

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1st Mention of that problem I have heard myself. Short of a positioning problem of the shift selector it would be an internal fault I would say. PARK and DRIVE is ok might mean its not the position.
 

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When you drained and filled the tranny, you actually drained only about half of the fluid. You then mixed half remaining OEM fluid with half non-OEM fluid. Many of us have completely flushed the transmission and refilled with all new aftermarket fluid and have seen no ill effects. Mixing dissimilar fluids seems to be what causes the problem. This is one reason I, and others, are adamant about using OEM stuff.

Get the transmission completely flushed of all the mixture and fill with new fluid. If you still have problems after that, I feel for ya, especially at the Christmas season.

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I'd be more than shocked if doing the fluid would fix that being reverse and a low speed gear in all. I don't disagree with your mix analogy and I used Castrol spec'd fuild in mine having had good experiences with their tech in other vehicle being SPIII certified as well.

Whatever has gone wrong is not going to get fixed with fluid change at least not now. It may have caused it but switching won't fix it. Also looking back up at the early post. Gear incorrect ratio code is not going to be fluid related. Maybe a solenoid or some valve stuck.

Call some shops familiar with SF trannies and see what they say. Otherwise you would end up with which ever one you go to saying you need a rebuild. Yeah it will fix it but overkill to fixing the problem. IMO that is what I would do.
 

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I've had some trouble shifting in reverse on my 2012 SF. I'm also thinking about changing the tranny fluid. Let us know if that solves the problem. Also, what brand fluid did you use?
 

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Thought I would check for any trouble codes, though the check engine light is not on. It turns out there is a P0734 code, "Gear 4 incorrect ratio." Is this more trouble, or does it somehow relate to the slipping reverse gear? Thanks.
It may not have anything to do with fluid.Look at post number 4 in the link below.There is a code there similar to yours. It may point you in the right direction.Good luck.

http://www.hyundai-forums.com/lf-2015-sonata-i45/607346-software-update.html
 

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I've had some trouble shifting in reverse on my 2012 SF. I'm also thinking about changing the tranny fluid. Let us know if that solves the problem. Also, what brand fluid did you use?
If you only do a pan drain, refill with OEM Hyundai fluid. As said above, it seems that mixing fluids can lead to problems. If you do a complete flush, the mixing issue goes away. In that case, there are many aftermarket fluids that will work. Valvoline MaxLife seems to be popular.

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