I had the same problem. It was the rear O2 nit that failed. It does set a
code that will show up.
If anyone needs an O2 sensor, by far the cheapest place is O’reilly auto
parts. It is the SAME Bosch sensor available elsewhere, at about half the
price. My Elantra has 90K and the O2 and a wiper blade are have been the
ONLY problems,
Jim