Hi, my name is Melissa & I have a 2003 Hyundai Elantra GLS that I bought used 2 years ago. I am hoping that someone can help me with the meaning of these codes that the work was already done; I just don't remember what the diagnosis was for these codes... they are:
#1) P0133
#2) PO455
#3) P0304

I have since purchased a FIXD Diagnostic Coder & ran a diagnosis @ 3 weeks ago & came up with the following code:
#1) P0420 - Warm up catalyst efficiency below threshold Bank 1.

There are 3 things that can cause this code; they are:
#1) Faulty oxygen sensor (could this possibly be the
downstream sensor?)
#2) Leak in the exhaust
#3) Faulty catalytic converter

I'm not positive, but I believe there are 4 oxygen sensors on my car & I have already had 2 of them replaced & the catalytic converter replaced all in @ the last 6-12 months. Has anyone else had code P0420 come up & what did you have to have done to get it fixed? I have poured more money into this money pit in the last 2 years than what I actually paid to buy it... I am sooo done with this car, but I can't afford to purchase another vehicle at this time!!!

 

Unfortunately, a, or, the code(s) don't always tell you which part is defective or exactly where the problem lies.
The code(s) only provide direction as to which system is having a problem.
The rest depends on the person doing the checking. If they don't fully understand how the system is supposed to work , what they usually do is start changing parts.
Some problems can require some special equipment and knowledge to find and correct.
I suspect wherever you are taking your vehicle for diagnosis and repair is coming up short in that area and you are paying for their schooling.
Find a shop hat has a good reputation and experienced people. You may pay a little more, but you won't be paying so often.

 

P0133 O2 sensor slow response
P0455 Evap emission leak
P0304 Engine misfire in cylinder 4
Can be caused by vacuum leak, exhaust leak, O2 sensor, etc

Yes, that is correct.

Four cylinder engine has one upper O2 sensor and one downstream O2 sensor.
Unless they put in a v6, there are only two O2 sensors.

What I suspect is that the mechanic used after market O2 sensors which may cause this P0420,issue.
O2 sensors are not cheap. Also, heater circuit may be faulty which can cause O2 sensor to not accurately measure the exhaust.
There can also be an exhaust leak which will throw off the O2 sensor readings.
Have heard of some people who put in an anti-spark-plug fouler. It essentially cheats the system into thinking everything is alright.
I don't recommend this as it can cause other issues.

I bought a 2001 Elantra GLS back in 2011. After five years, spent more money fixing it than if I would have bought it new.
It got to a point where I could not afford to fix it anymore. Sold it off .. cheap.
Then bought a 2007 Honda CRV. So far so good.
I truly believe that Toyota and Honda are better value and more reliable.
But, that is my opinion.