You only have one upstream oxygen sensor on the Elantra, so "upstream
heated 02 sensors swapped"
is not a valid diagnosis.
This code indicates the computer has been supplying more fuel than it
thinks it should have to based on the other sensors' readings. This is a
(somewhat) common problem with 2000-2001 Accents It's okay to reference
the tsb on this issue (01-36-022), but don't use it for diagnosis because
the values will be different than those on an Elantra. If you want a
better understanding of what the code means, tsb 98-36-001 provides a
reasonable amount of information.
What you want to look for is unmetered air entering the engine (meaning it
leaks into the intake without having to go through the air flow sensor).
The primary ways this happens are leaking pcv hoses, intake hoses not
fully attached to the air flow sensor, and intake hoses torn. If there
are no issues with unmetered air, then you probably need a new air flow