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.
Car Problem Reports
Hyundai Elantra Transmission Won't Shift/CEL Due to Failed Speed Sensors in Transmission
Hyundai Elantra Problem
Average Mileage: 82,042 mi (21,200 mi - 142,000 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.
Engine will rev up to 4000+ before shifting. Sometimes it will downshift at freeway speeds.
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, 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.
Visitor, 2002 Hyundai Elantra, 142,000 mi
Same problem with AT shifting poorly, stays in 2nd gear much longer, sometimes can manually shift with the AT and watched the youtube videos on replacing the tranny sensors. Got both off eBay for under $50. deciding if I want to do them myself or pay someone to do it.
Visitor, 2001 Hyundai Elantra
same problem with speed sensor
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, 2004 Hyundai Elantra, 67,000 mi
Faulty shifting, check engine light comes on then electrical system dies altogether. This happened 6 months ago and the input sensor was replaced. Happened again yesterday.