If the automatic transmission does not shift properly and the Check Engine Light illuminates, the speed sensors in the transmission may have failed. The sensors are simple and inexpensive to replace.Google+
Car Problem Reports
Hyundai Elantra Transmission Won't Shift/CEL Due to Failed Speed Sensors in Transmission
Hyundai Elantra Problem
Average Mileage: 82,976 mi (21,200 mi - 150,000 mi)
Visitor, 2002 Hyundai Elantra, 100,000 miRookie
Car has had this problem for a while now. Being a broke college student I freaked out and assumed it needed a new transmission, so I sort of swept it under the rug and kept driving while it let me. It doesn't happen all the time, seems to be the most when I'm hovering around the speeds where it would normally shift gears, especially when my speed is changing a lot (i.e. in traffic). For the most part, when it happens I pull over, wait for it to reset, and usually it works fine. Sometimes it still stays stuck when I start driving again, so I repeat the process. Eventually got myself to research it after almost a year (and driving it wayyy less than normal) and from what I've read it seems to be bad Input/Output Speed Sensors. Another thing I gathered from what I've read is that a LOT of people have had this problem. Manufacturer recall would be nice.
Visitor, 2001 Hyundai Elantra, 125,000 mi
I bought the car used, it shifts hard and then stays in third gear. Turn off car, reset eng. lite and resume. Replaced input, output sensors and Idle Speed Control . Not hard to replace. Got them on Amazon.com for $126.00 and installed them myself. Problem solved.
Visitor, 2011 Hyundai Elantra, 65,800 mi
Bought my car used (previous 1 owner) in early 2013 as soon as I was over 60000 my car started with a jerky jolting problem when the car would shift. It gradually got worse and check engine light came on. We took it to a local dealer and found it was the transmission sensor. After paying almost $700 to have this fixed we found that this is a very common problem - I'm surprised Hyundai has not recalled on this yet.
Engine will rev up to 4000+ before shifting. Sometimes it will downshift at freeway speeds.
Visitor, 2004 Hyundai Elantra, 50,000 mi
Car will downshift at higher speeds, runs sluggish. Sometimes will stall at red light. Check engine light is not on, so dealership says nothing is wrong. I don't like to drive on the freeway anymore, it's frustrating.
Visitor, 1999 Hyundai Elantra
will not change into any other gear.The rpms shoot up and it runs sluggish, just will not go from drive to 1st or 2nd at all
Visitor, 2007 Hyundai Elantra, 53,800 mi
downshifts to 3rd and stays there until a stop and restart
Visitor, 2005 Hyundai Elantra, 61,000 mi
Check engine light came on and shifting problem occured. Took it in to dealer and it was sensor problem, got sensor replaced @ $290.00 (not covered by the big 100,000 mile bumper to bumper/10 year warranty).
Visitor, 2009 Hyundai Elantra, 21,200 mi
Took the Elantra in to the service area to be looked at for the transmission not shifting right while going down the road. Shifts hard, slams into gear, gear shifter hard to take from park to another gear. Dealership told use they never have transmission problems with the Elantra. And after talking with manager saying that something was wrong and a test drive with my husband, the manager said there is defiently wrong with the transmission. So we now have to get a brand new transmission for our year old car.
transmission jerk on initial start and check engine light on.