2001 elantra lack of power

could be a failing cat converter and the code would be refering to a roblem heated oxygen sensor (HO2S) in an exhaust manifold before the cat converter
have an exhaust shop check the cat converter as they do break up internally , rattlr around and in some instances can block the exhaust system

code p 0420 refers to cat converter system bank 1 --efficiency below threshold====causes --cat converter===wiring ===HO2S ( heated oxygen sensor in the exhaust manifold before the cat converter
would not waste money as the ecm does 60 tunes up every minute and adjust whatever is needed to maintain optimum tune for the engine
replace the heated oxygen sensor to allow the ECM to maintain the tuneup and rep[lace the cat converter if it is faulty

your cat is plugged remove it and replace with straight pipe or replace cat

if you replaced the manifold because it was cracked the look for a plugged cat convertor..the cracked manifold was probably letting the exhaust escape..now it builds too much back pressure

I am having the exact same problem with my 2001 Hyundai Elantra. I was told that the engine code that comes up when the sensor is on is called "Absolute Pressure In The Manifold". I was told by a "Hyundai Specialist" that when that code comes up it usually means that the seal on the manifold is Fuct. When my mechanic checked that out for me he had noticed that my manifold had 4 cracks in it(can varify cause he showed me). He replaced the manifold and gasket. The car ran fine for about 2-3 days and since then the problem has since come back and my check engine light will no longer go on/off it only stays on. Because of the check engine light always staying on I have noticed that on many occasions my car will hiccup many times in a row. The only way to stop the hiccup I have noticed is to take my foot right of the gas for approx. 2-3 sec then car will continue without hiccup(till it happends all over again). All in all I have been having this problem now for approx. 1 year and if there is somehow someway it could be repaired I would appretiate it.

There is a vacuum line that runs under the manifold, it seems to be a very common problem.

Go to autozone, have it scanned with a OBD2 scanner and come back and post the trouble codes, also have the autozone guys look for a bad vacuum hose that comes off of the manifold, they might find it right away.

Sounds like your cat converter has become clogged from diesel....take it to a smog tester to see if cat is bad

Well 1 way to tell if it is the cat un bold the exhaust at the manifold or try and remove the oxygen sensors in front of the cat if it runs fine then you know that the cat is pluged

code p 0421 refers to warm up cat converter bank 1 --- efficiency below threshold ===causes ---cat converter---wiring ---HO2S (heated oxygen sensor )
there are not 4 oxygen sensor as you have been told but just 2 oxygen sensors located after each of the cat converters
The other 2 are heated oxygen sensor which are located in the exhaust manifold or exhaust pipe before the cat converters
heated oxygen sensors do a completely different job to the O2 sensors in that they report to the ECM, the combustion gas mixture so the the ECM can adjust the mixture for max performance
The oxygen sensors (O2) read what is left after the cat converters have done their job and report to the ECM so that it can fine tune the mix
IF you didn't replace the correct cat converter then that will make the code come back
If you just replaced the O2 sensor , well that doesn't even figure in the problem