How can I tell the difference between my catalytic converter going or an O2 sensor?

Question

My check engine light is on so I brought it to Autozone and they plugged it in and I got 3 codes (1st one says possible Catalytic Converter failure, or O2 sensor p0420 is the code I believe). The other two codes I was told aren't big things (like a simple oxygen hose not plugged in all the way or a loose wire).

I have a 2003 Dodge Neon SXT that had been sitting for a year before I bought it.
I put in Cataclean into my gas tank to try and "clean" off the sensors and the catalytic converter but the check engine light is still on! :(

I am trying to make sure if I would need to just replace the catalytic converter.
I was told catalytic converters last practically a life time and typically don't need replacing so that is why I am asking you guys to help me out and let me know if you think I need a catalytic converter or not! Would the codes read directly to the front or rear O2 sensors if those needed to be changed? Therefore I DO in fact need a new catalytic converter?

Please help, it is very confusing and I really want to get this check engine light off as soon as possible!!!

Answer 1

First, i want to explain what is the code P0420 means?



The catalytic converter has an oxygen sensor in front and behind it, when the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.



A code P0420 may mean that one or more of the following has happened:



(1)Leaded fuel was used where unleaded was called for

(2)An oxygen sensor is not reading (functioning) properly

(3)The engine coolant temperature sensor is not working properly

(4)Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe

(5)Retarded spark timing

(6)The oxygen sensors in front and behind the converter are reporting too similar of readings



Some suggested steps for troubleshooting a P0420 error code include:



(1)Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.

(2)Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).

(3)Inspect the downstream heated oxygen sensor (HO2), replace if necessary.

(4)Replace the catalytic converter.



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Answer 2

You cannot know without the proper tools and equipment; that's why programs like this "answers" thing exists. You can pay for a diagnosis or take a chance on putting in parts and hope it works. You are correct in thinking that replacing parts does not always fix a problem, sometimes a connector, wire, or blown fuse is the problem as well. If you had big, negative changes after having installed the primary o2 sensor then it is likely you got a bad one or damaged it putting it in. The rear o2 sensor is only there to test the catalytic converter, it does not alter the fuel system.

Answer 3

Before you go springing for either a new O2 sensor or catalytic converter- get the exhaust system checked out! Some times a cracked manifold or a blown exhaust gasket will cause this problem. All this code is saying is that the drop of emissions levels is not what the ECM expects. If air is getting into the exhaust it throws everything off. If you hear a ticking under the hood, suspect an exhaust leak. Some times these leaks sound like tappet noise.

Answer 4

It is unlikely to be the catalytic converter.



I would do the simple ("not big") things first. Check to see if there a loose wire or oxygen hose leak.



Next, just change the O2 sensor (most likely problem IMO) - sensors in general have a real bad habit of going bad and are not real expensive (in the neighborhood of $30 - unlike catalytic converters - about $100). I've had several sensors go bad and not had one of the expensive problems they are there to tell you about.



Those three things will not cost much and the problem is more than likely one of them.



If those fixes don't cure the problem, guess what? You haven't spent much (not as much as taking it to a mechanic) and you pretty much know what the problem is.