Catalytic converter?

Hey guys,

I replaced my catalytic converter with a cheap one about 6 months ago, it was stolen.
Anyhow I have recently failed emissions twice. Once before fixing the cracked exhaust header I found and once after.

I have failed for high hydrocarbons which did come did come down some after the header was fixed. I also failed due to high Nox. This remains almost identical the second test.


I'm trying to find out what else I should be doing to diagnose the failure and if this seems like I just need a good catalytic converter?

If I helped two years ago I passed my emissions test with no issues what so ever.



99 jeep cherokee 4.0 automatic



Thank you all for any help you can give.

Things to check:

1. Make sure all O2's can respond to a Rich or Lean Condition (they are telling the truth)
2. Zero exhaust leaks ( No smoke machine, no problem, hook a shop vac up to the tailpipe and set it to blow air, spray the exhaust system with soapy water)
3. Make sure your ECM has fuel control, ie the total Fuel Trim = Short + Long is under 10 percent ( if the trims are 10% and better this needs to be addressed first) Every engine is different, but I think the grey area is between 10-13 Percent, over that there is more than likely an issue. we want to avoid rich conditions which can destroy a cat really quick)
4. Not burning coolant or oil.

I included a Scan Tool Cat efficiency test I did for a customer. Clearly, there was Zero Oxygen Storage Capacity. But it is a simple enough test with a very warm engine. Three quick consecutive throttle snaps and watch the recovery of the Post Oxygen Sensor.



Unfortunately, it is well known that the aftermarket Cats have a lot less precious metals in them compared to OE. Hence poorer performance and life expectancy.

https://imgur.com/a/3h9PNKG
After hooking up the Vacuum I found two areas of exhaust leaking. The first was a crack on a weld of the header. The second was where the down pipe mates to the header, this is better now, but still needs a new downpipe ill have to weld on as the current has ovaled out. This leak was far smaller than the crack.

If the above link works, its a picture from a cheapo bluedriver scanner, but everything was registering and would change with the as you gave or let off the gas. I went ahead and welded on a new catalytic converter and it passed emissions!!!

The jeep is well within the testing limitations, however as a concern, should I worry about replacing any of the o2 sensors having driven on these conditions for some time before finding out there was an issue?


Thank you very much for the guidance, I'm relieved that my vehicle is road worthy again!!!

Your only out of the woods for a while, looks like your Total trim is -13%. You have a Rich condition occuring. The PCM is subtracting fuel.

If you are truly Rich, this will kill the Cat.

What are the load pids Calc Load and ABS Load doing at idle and Wide open Throttle?

imgur.com/a/GYPj7e7

This is the latest test from passing emissions.

You said its a rich condition but the PCM is taking away fuel? I'm confused, isn't that lean?

I'de have to get another scanner to see what the other readings you aksed about are doing. I sent the two I had back because they didn't read my intermittent SRS airbag light ( separate ongoing issue). Ill see what I can do though.

Based on the emissions test passing do you think it is still rich?
I plan to put New O2 sensors in to make sure the PCM is getting accurate readings.

Those numbers mean nothing to me, I only understand PPM, Parts per Million when it comes to exhaust gasses.

I'm in Northern Ontario, we don't worry about that testing....yet.

You said it's a rich condition but the PCM is taking away fuel? I'm confused, isn't that lean?

Your PCM is subtracting fuel because of a rich condition. It is still in fuel control. If it wasn't subtracting the fuel your mixture would be too RICH. Whether it be a lying sensor, leaky injectors, too high fuel pressure, EGR issues, etc. would remain to be determined.

During a LEAN condition, you would have positive trims and be adding more fuel, because the PCM "thinks" there isn't enough.

The Bluedriver can collect the required data and also save it as an Excel file. You can then post this file for analysis. (Post a shared drive link, ie Google Drive.)

PIDS of interest:

ECT
IAT
BARO
MAP
ABS LOAD
CALC LOAD
All Fuel trims (Long and Short Term)
RPM
ALL O2 voltages (B1S1 and B1S2)

Have the engine warm and take a nice 10 minute trip, and frequently, at least three times accelerate hard until max RPM achieved.

Post the Data from the Bluedriver here.