Help - how to diagnose a bad catalytic converter?

Ok, need some help with this please: 05 KJ, 280k miles. Brand-new spark plugs (the correct copper type) and refurbished all 6 coils at the same time. Had a P0172 code pop up. Cleared code and ran some injector cleaner through with a full tank of gas. Code returned along with p0456. Bought new gas cap as I found the old one had a split gasket. There is loss of power on hills but on flat ground it's running just fine. My shop is trying to convince me that this means a bad catalytic converter. They won't even check fuel trim. I have not had a p0420 etc. code. I have tapped both converters with a mallet and nothing is rattling. I have used a thermometer and got 430 deg. pre-cat and 450 post-cat on passenger side, 410 deg. pre and 460 post cat on driver side. My scanner is too cheap to provide live details about O2 sensor performance, etc. Does anyone know the correct temps for a working cat? Is it supposed to be hotter on one end or are the temps supposed to be similar pre and post? Answers seem to vary depending on where you look ...
Don't want to throw a $800 part at it if it's likely something else causing the issue....

Edit: Somehow this did not end up in the General Discussion as intended. If someone could move it, I would be very grateful. Thanks.

First, get yourself a live data-reading scanner. They are quite inexpensive these days and are a tiny fraction of what a shop will end up charging you for (many times questionable) work. The fuel trim values are the 1st thing I look at when diagnosing any CEL-related problems.

Despite many youtube videos to the contrary, taking their before and aft temperatures is not a reliable way to determine their efficiency. Nor is your idea of hitting it with a hammer. There are PIDs relating to catalyst efficiency percentage on live data scanners that you can look at to see how well they are performing. They basically just translate what the rear O2s are reporting.

P0420/30 codes are indeed the only ones that confirm a cat is no longer doing its job acceptably. Though doubtful, it might be possible those codes are "in the winds" and might pop up once you deal with what's causing the the other 2 codes you mentioned.

P0172 is, as u probably know already, is system rich bank 1. The causes for this are numerous. If the upstream O2 is reporting correctly, continuing to drive the vehicle without addressing the cause could lead to cat failure.

P0456 is a small EVAP leak and could be related to the P0172, as the purge solenoid could be stuck open, which would cause a vacuum leak and resulting rich trims.

Thank you for the detailed answer. I was basically throwing a hail Mary with the described tests since this problem just presented itself on Friday and I have a long road trip coming up tomorrow and won't have time to do anything more detailed until I get back. This vehicle is my daily driver so I have no choice but to drive it tomorrow.
I am also a complete newbie at diagnosing engine problems based on scanner data. The concept is one thing, but my confidence in knowing how to interpret the numbers is low. This Jeep has had very few engine issues!
Most of my DIY repairs have been based on visible problems with various parts, suspension, hoses, leaky radiators, reservoirs, pan gaskets, water pumps...

Lots to learn!

The upstream O2 sensors are original, the downstream sensors had a code last fall and were replaced then. I guess replacing the remaining ones would be a prudent first step?
The 0456 code has popped up a couple of times before over the years and it was the fuel tank cap each time, the engine light turned off by itself within a couple of miles after installing a new one. Here's to hoping that maybe that issue was resolved with the new cap...

I will be looking into getting a better scanner when I get back.

I just replaced both of my cats. I purchased the whole Y pipe with them on it from Rock Auto for $480. The pipe was an OEM fit also.

Mileage says it is time to replace the cats. I hate wasting money buying the wrong part. You WILL need to change the cats soon, so if finances allow, maybe now is a good time.


ANYBODY OUT THERE HAVE SAME SET-up aND WANT TO DO A TEMPERATURE GUN TEST FOR ABOVE AND POST PRE AND POST CAT TEMPERATURES? WOULD BE NICE.

I don't know if this applies here, but this might be a nugget of information all y'all might like to know: I used to have a '91 YJ (I'm pretty sure this is the vehicle this happened to) and the innards of its catlytic converter came loose and turned 90°, then wore itself into the discharge hole. The engine would idle fine and could rev up to about 2K RPM, but no more. After that, the exhaust pressure forced the catalyst toward the discharge hole and cause the engine to stall. I remember the mechanic I took it to stratching his head and saying, "That's a new one on me."

Well, 460 miles later and engine light is still off! Uphill sections climbing from sea level to 4,000 ft. were a struggle and I ended up turning OD off and that improved things considerably. Engine ran normal/cool with the temp gauge needle just shy of vertical. Now if I also make it back on Monday things will be looking up!
One thing I noticed was that after each rest stop and then restart, the CEL would sometimes be illuminated for a couple seconds along with the other usual dash lights when the key was turned and sometimes it would not illuminate at all. Any significance in that?

budget76 Yes, I most likely am on borrowed time, lol. Original engine and transmission. A little bit of oil consumption that will have to be addressed at some point but otherwise in good shape I think... It would be awesome if someone could check their temps around the converters. Mine is the 3.7l 4x4. I measured after 15 minutes of driving in 95 degree ambient temperature. Since my results are not consistent between the two sides, I am thinking one or the other side is bad?
LONGJEEPOWNER- Thank you for the pricing info. Is this the setup where you buy several individual parts? Are they easy to put together? I also looked at the all-in-one offering (single part for $700.)
DHBirren2- That is interesting. Will keep this in mind since my other car is a '94 YJ - 180k on original engine and manual transmission. Currently needs a new differential gasket. Projects stacking up....

Catalytics can last the life of a vehicle if the engine is kept in proper tune. You shouldn't replace them w/o some hard evidence that they're bad. As I said before, a live-read data scanner will be your best bet to see where the cat efficiency is at. Yes, there's a learning curve on how to read the tools, but I can try to help you through it. It beats depending on ******* mechanics who 9x out of 10 don't know what they're doing yet see no issue with charging people through the nose as they guess which part should be replaced next.

If you're worried about a restriction in the exhaust, remove both upstream O2s and see if it drives any better. A simple vacuum gauge can be used to determine if there are restrictions, as well. The honeycomb structure inside the cat can flake away, like DHBirren mentioned, if the cat temp goes critical- usually do to a misfire or excessively rich mixture that went unchecked.

If the upstream oxygen sensors are original, replacing them is waaay overdue. Make sure to only buy NTK brand and make sure to get them from AutoZone or the like. I would avoid getting them from Amazon/Ebay/Etc.

Ksat said:

Catalytics can last the life of a vehicle if the engine is kept in proper tune. You shouldn't replace them w/o some hard evidence that they're bad. As I said before, a live-read data scanner will be your best bet to see where the cat efficiency is at. Yes, there's a learning curve on how to read the tools, but I can try to help you through it. It beats depending on ******* mechanics who 9x out of 10 don't know what they're doing yet see no issue with charging people through the nose as they guess which part should be replaced next.

If you're worried about a restriction in the exhaust, remove both upstream O2s and see if it drives any better. A simple vacuum gauge can be used to determine if there are restrictions, as well. The honeycomb structure inside the cat can flake away, like DHBirren mentioned, if the cat temp goes critical- usually do to a misfire or excessively rich mixture that went unchecked.

If the upstream oxygen sensors are original, replacing them is waaay overdue. Make sure to only buy NTK brand and make sure to get them from AutoZone or the like. I would avoid getting them from Amazon/Ebay/Etc.

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I will definitely do the upstream sensors. NTK is what I used for the downstream sensors and have not had any more codes related to them since the replacement.
Have not had any issues with misfires and the converters do not "look" red/overheated.
The CEL came back on today, after about 600 miles with the new gas cap, for code P0456 again, so I guess the evap leak isn't the gas cap after all.
Now I am waiting for the next code to return I guess...
Do you have a make or model number for a scanner that you can recommend? Or a list of functions it definitely needs to have for more in-depth analysis? My budget for this would be no more than $150, $100 would be better... Are the scanners from Harbor Freight any good? I definitely want to find out what is really going on as soon as I get back home.

For the P0456, time to tighten the clamps, and if possible replace the rubber hoses in the evap system. You could do proper testing of sub-components with a vacuum tester/pump thereby reducing what it may be / is. If you can rule out some components, then you will by definition find the faulty component. Do not play the replace part and hope game. Sometimes it is ok as you end up with new parts, but ....

Do you have a make or model number for a scanner that you can recommend? Or a list of functions it definitely needs to have for more in-depth analysis? My budget for this would be no more than $150, $100 would be better... Are the scanners from Harbor Freight any good?'

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Sticking with a brand name might be your best bet when it comes to a mid-level scan tool, though I would stay away from the Innova brand, personally. I've had good luck with my Autel 519 and, although it's probably over 10 years old now, it's still pretty useful (looking on Amazon right, I see one for $60). I would avoid anything coming out of Harbor Freight for scanners as the last time I checked they were a lousy deal overall. I've heard good things about the Launch brand, but don't get the feeling from their Chinglish website that tech support is that great. Actron has been around a long time, but I don't know much about them. There's a plethora of no-name brands out there that would seem to offer a lot for the money, if you believe the ads, anyway. They might be worth trying out if the seller offers free returns and you can send it back without incurring cost.


If you don't need stuff like airbag or ABS functionality, the price will be cheaper than if it has those. Whatever you go with, make sure it says it has the ability to read live data, access something called Mode 6, can display all generic OBD2 codes (most I think do), graphing ability, can display monitors that are complete/incomplete (I/M readiness status).


If you have an Apple or Android device, a possible less expensive option is to purchase an ELM327 device (allows you to connect the device to your car) and download an app like Torque/Torque Pro and use that as your scanner. If you decide to go with this option, spend some time reading the reviews before you buy the ELM device, as I've heard some are more capable than others.

Regarding the EVAP leak, A smoke test is the defacto way to find out where one of those is coming from. You can pay a shop to do it, or, If you're handy and already have an air compressor, you can build one for around $30 or so (I'm actually in the midst of doing this myself, as bonified machines are crazy expensive).

Is the leak preventing you from passing inspection?

Passing inspection is not an issue currently, thank goodness. But the recurring engine light bugs me, lol. Always worried that it might be something more sinister every time it comes back on.

After looking everything over and not finding anything obvious, I broke down and paid the shop $69.99 for smoke test. (I don't own a compressor.) No obvious leaks found that they could show me. So they suggested replacing the natural vacuum leak detection pump. Until I mentioned that it was replaced 25,000 miles ago after a "large emissions leak" code. Then they suggested the fuel evaporative canister. Sounds like a guessing game to me. Ugh! On the other hand, many parts on this Jeep are original and 280k miles old so maybe new parts wouldn't be a bad thing.

The p0172 "running rich" code has not returned after now over 1,000 miles driven over the past week. But I read somewhere it takes 40 starts with the problem present before it throws the code again? I am still doing some more research and then getting a scanner. Thanks for the info and model suggestions - the cost is doable.

The vacuum system is easy to diagnose using a hand vacuum pump. You can rule out canister, lines until the last thing (always the last thing) does not hold vacuum or else solenoid does not work.

There's s pretty simple trick, of sorts, to get your Lib to pass inspection if it has an EVAP code. That is if your state allows for at least one monitor to be incomplete prior to the test due to the age of your vehicle, I know NY allows for one incomplete on my '06.

The p0172 "running rich" code has not returned after now over 1,000 miles driven over the past week. But I read somewhere it takes 40 starts with the problem present before it throws the code again

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Not sure where u heard that, but the 40 starts statement is a bunch of malarkey.

Sorry for the long lapse here - had to deal with a family emergency and put another 1,500 miles on the Jeep in the past couple of weeks - with the check engine light on... I checked the codes daily, it still drives fine, gas mileage is steady at 20/21 mpg while driving mostly rural roads, and no new codes appeared on the list.
I am purchasing a better code reader tonight and will be back on here to see what the results can tell me.
Thanks for all the suggestions so far!

Finally received the new code reader today! Found a decent price on a Autel ML529. Now where to even start, lol.
I figured out how to retrieve the codes with it but everything else is new to me.
Will have the day off on Sunday and start the testing. I hope I can read up on this between now and then. Any advice on what/how to test would be awesome!

Ok, read a lot but I am still lost. Ugh.

Codes retrieved at this time are:
P0457 fuel cap loose (have replaced caps twice in the past 8 weeks, code keeps coming back)
P0456 very small evap leak
P2097 post cat too rich bank 1 (both post cat O2 sensors were replaced a few months ago)

I tried to read data for O2 sensors but get "function not supported" error (vehicle was at operating temp and running). The scanner does not identify if this is mode $05. From what I read for this vehicle it would have this function scanned under Mode 6 since it is CAN?
Under what I am guessing is mode 6(??) (Live data) there are acronyms that contain "O2" and "SHTFT" and many others. Live data was showing fluctuating numbers but I obviously don't have any idea what I am looking at.
Which live data (acronyms) do I need to select and read and what should the numbers be?

Having p0172 and 175 - lots done to this point and still not right.
New data- cold start coolant temp does read on the scanner at 60 steadily runs up to 79-80-113 seamed normal. Front 02’s randomly ran .100-.324-.845-.730 etc on warm up. Rear 02’s .840 .860 which is similar to what they run when it’s all warmed up. Could this be bad cats? Holding exhaust before the cat showing rich conditions?
I’ll search for wiring diagram and try to test from the pcm to the 02. Maybe Google knows the pin locations.

Any help is appreciated she’s almost right.

As you're learning, desertdriver, there's a steep learning curve to understanding what all the data means. If it's any consolation, I was in the same boat only a few short years ago. If you have time, there are some good youtube videos from "GoTech," "Scanner Danner" and others you can check out, which might help bring you up to speed on what all of it means. Take notes, as there's a lot to digest.


To briefly help answer the questions you posed, STFT (short term fuel trim) and LTFT (long term fuel trim) are collectively referred to as your fuel trims. STFT should quickly switch between ~5% positive and negative at idle when engine is warm. LTFT should be as close to 0% as possible. Fuel trims are perhaps the 1st thing you should check out when looking under live data, as they can help tell you which direction to take with the diagnoses.

The graphs for upstream oxygen sensor voltage outputs should look sinusoidal. This is at idle with engine warm, again. If they do, then you would typically perform a fuel enrichment and en-leanment test to see if the STFT responds the way they're supposed to. Enrichment usually means adding propane into the intake while looking for the trims to go negative. En-leanment means pulling a vacuum hose while looking for the trims to go positive. You could also look at the upstream voltage output changes instead of the trims if you wanted to have a more direct approach about it, but it's usually just easier to do the former.

P0457 fuel cap loose is directly related to P0456 very small EVAP leak. A loose or bad gas cap is probably the biggest cause for a small EVAP leak, so the system is just suggesting that as a fix. Small EVAP leaks can be hard to find, even with a smoke machine. There are some tricks to help make the escaping smoke easier to spot, including using laser pointers and such, but generally the smaller the leak, the harder it will be to find. If I were you, I wouldn't worry much about the leak, as it shouldn't affect driveability or gas mileage any. If it turns into a large EVAP leak down the road, that's a different story, but it should be easier to spot on a smoke test by then. If the CEL being on bothers you, erase the code then keep the gas tank maybe 3/4 or more full (or 1/4 or less empty), as the EVAP system won't run its test under those conditions and the code shouldn't be set.

P2097 post cat too rich bank 1 is an interesting one. I'll try googling that to see what I find. Perhaps, in the meantime, you can post what your STFT & LTFT % are (at idle when engine is fully warm) here, since that may offer some insight.

Mode 6 is good for reading misfire counters for each cylinder. It can be helpful when pinpointing which cylinder is misfiring, especially when the PCM hasn't been kind enough to trip a code letting you know which one it is yet (P0301, P0302, etc). Some PCMs don't throw a misfire code until the actual misfire count gets ridiculously high, so Mode 6 can be invaluable in those situations.