2001 Sonata No shift

My sonata has now all at once stayed in 3rd. gear and not coming down to low when I stop!- the engine light came on. Lasted about 30 minutes and the the light went off and shifted beutiful for a week or two. then all at
once same thing! Later that evening drove to church no problem! light still on this time.Can't get into dealer for a few days!With a ATF filter on top and a plug on the bottom is there any reason I can not go ahead and change fluid? With SP 3 of course.
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There's no reason you cannot change the fluid, but there's also no reason to expect that it'll do anything to resolve your problems, either.
The transmission computer is putting the transmission in failsafe (3rd gear only) and requesting the check engine lamp to be turned on because it's detected some sort of problem. Each time you turn the car off and on, it'll shift normally until it sees the problem again. If you're doing mostly stop-and-go driving, you shouldn't continue to drive the vehicle while it's in failsafe. Continued starting from 3rd gear can damage the transmission.
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