My XJ is running kind of rough. Not horrible but bad enough that I feel it and want to fi it. The CEL light throws the P0420 code. So i know what the code means. But my cat is somewhat new and I know these things are built to last for quite a while. I have an oil leak (it's a XJ) and I think I remember reading that oil can cause a cat to go bad.

Question: Are there any ways to check/test a cat? To know for sure if it is working. I mean testing the cat itself. I understand that if the resistance of the downstream O2 sensor reads the same as the upstream O2 sensor the cat is not working properly, but is that the only test.

I hate replacing parts and would rather diagnose/repair the problem than treat the symptoms. Any help or prior experience would be helpful.

 

You might want to check/gap your plugs first just on GP. Something that needs done from time to time anyway, then you can rule that out.

I had a clogged cat that caused really poor power. Found it by running it with the header flange bolts loose for a minute. Night and day on this one.

 

Yeah, I think that would be a good place to start. I had this same problem before when I did my tune-up over the summer and it turned out I had one Spark plug with a bad gap.

By header flange bolt are you saying that you loosened up your exhaust bolts and ran it that way? So basically the the exhaust never made it to the cat to cause any poor performance.

 

Right. Noisy and a potential fire hazard, but it told me in 12 seconds the exhaust was restricted. I seem to remember the "repair" involved a torch and a long drill bit. Anyway the symptom was it was just flat gutless. Maybe someone else can chime in on testing one, that's all I've got. Let us know what you find!

 

Ok so the spark plugs were all good. I tried seperating the exhaust at the collector flange to see if there was any obvious changes from a sealed exhaust to one that had an outlet before the cat. I wasn't able to get a clear determination because I didn't want to disconnect the flange completely and I couldn't get a good seperation. However, it did seem that the rough idle or skipping was still present. Seems that if the exhaust can bypass the cat and the problem is still there, it is another problem? Is my reasoning wrong?

Possibly the tps? I seem to remember reading a way to test that sensor so maybe I will research that next.

Any comments or suggestions?

 

you could have a much cheeper issue here. you need to get ahold of a scaner that can show you live data, and look at your fuel trims, and voltages the long term and short term trims. should be under 10% combined. the closer to 0 the better but not commen. alot of times a cat code like that is thrown by a o2 sensor or a vac leak or even a over or under fuel issue. ive had alot of customers who have gone and replaced cats and have the code come back and wind up being an o2 sensor. alother thing to keep in mind are the o2 sensors switching fast enough or are the voltages moveing really slow after the cat. that would be bank one sensor 2 after the cat if the o2 sensor is moveing slow, it thinks the cat is bad witch could also be your issue. witch ever way you go with this make sure you compleatly look at everything good luck.

 

Superman, excuse my ignorance, but since I am not a professional in this area, I am not sure what you are suggesting. Are you saying that my O2 snesors may be what is causing the CEL? My understanding was that CEL code P0420 would not show if the O2 sensor is bad. Again, I may have misunderstood anything that I read. Secondly, the live read that you are talking about, is that a basic diagnostic machine. I know the scanner that the autoparts store uses is rudimentary and is good for identifying basic codes. So I would need to go to a certified mechanic or dealer to have this done?

I also got amuch better "exhaust leak" at the exhaust flange and still felt like it was running the same. So I think I need to check the engine somewhere. I am about spent on my skills at diagnosing.

 

I'm no help here, mainly just wanted to share my Cat story in response to your question. Still have plenty to learn on the earlier Renix Jeeps. It might be it's the first O2 sensor by the engine that helps control the mixture, and the one after the cat monitors cat performance.
You might think about fuel and air filters, again just on GP, something that needs to be done from time to time anyway. Check hoses and listen for vacuum leaks. You might check a few bolts holding the intake/exhaust manifolds, If you find some loose there is a whole procedure tightening the whole business. (they are known to loosen).......

 

ok im sry ill be more in detail. the fuel trims show how much or how little fuel you are getting through the engine. the o2 sensor is what picks that up. so if the sensor after the car it going bad it will miss read and give you a false code. intern having you replace the wrong part. and with a cat that can be verry expensive. a scaner with live data is reading the car as it runs not like a code that tells you what has already happend. you need to know what is going on whell it is going on. your best bet is to take it to a good shop that knows what they are doing and they whould have no problem telling you what the fuel trims are and if its your cat or you o2 sensor.

another thing is they exaust leak MAY be affecting the issue. as the cool clean air comes in to the exaust where your leak is its confusing the o2 sensor as well. this is a possability not a exact fix but its worth a shot.

and yes i am saying it may verry well be your o2 sensor over your cat. but i could be wrong thats where the trims come in good luck let me know how it turns out

 

Superman1456 you are pretty accurate with your statements but just to clarify, fuel trim is not how much fuel is running through your engine... It's a measurement of how much fuel is added or subtracted to keep the fuel air mixture closest to stoiciometric efficiency (14.7:1 air/fuel ratio) and this is determined by the oxygen content that is measured by the pre-cat O2 sensor. The pre cat O2 sensor sends this information to the computer and the computer changes the duration of the injector timing to correct the ratio. The second O2 sensor is just there to make sure the cat is doing it's job efficiently. A PO420 code is a "catalytic converter efficiency below threshold" fault code and is usually caused by a bad cat but could also be an exhaust leak or post cat O2 sensor. As for a bad air leak introducing cool air to the O2 sensor like you said... That's impossible because air doesn't leak in but the exhaust will leak out and the O2 will read less oxygen content and think the cat is faulty. The exhaust gasses are under pressure as it travels through the exhaust and this pressure is what would prevent ambient air to enter the exhaust stream.

 

There are "reverberations" in the exhaust that can draw in a little air. Every square mm. is not always under pressure. Not saying I have a clew if that could be related to your issue though!

 

Since you said your catalytic converter is somewhat new, the downstream O2 sensor is definitely the first thing I would check.

 

To check the cat - use an infra-red thermometer - run at 2000 RPM, let idle, measure temps in and out of the cat. Outlet should be about 100 degrees hotter coming out.

An exhaust "pules" - running from pressure to vacuum. That's how headers and "Pulse air" systems work. It can easily "suck in" fresh air - O2 - cvausing tne systyem to think it's too lean, so it richens up. Often a reason for poor mileage..

A small leak in the exhaust can set an O2 - or cat efficiency code.

But this one sounds like a vacuum leak - rough idle. A cat won't do that, neither will an O2 sensor or an exhaust leak.

Blow smoke through a hose into the intake manifold - engine off of course.
Fix wherever it comes out - including under the dash - heater/AC controls.

 

Lot's of good information here. I am pretty sure there is a small exhaust leak upstream of the cat. I remember seeing a black smearing at the connection between the pipe and cat. I am not home right now, so I guess this will have to wait untill I get home after the holidays.

Someone suggested blowing smoke throigh the system. Any suggestions on how to create smoke?